In the state of Montana , a renter fell behind on rent due to job loss.

The landlord sent them a letter saying they had 3 days to move out.

The tenants were trying to get help through some government agency..

On the 5th day the landlord changed the locks and locked the tenants out of the place while they were gone.

Is this legal to do?

DIY eviction?  99.99999% chance it wasn't legal.

Yes they haven't been served by anyone. Only received a handwritten letter, by mail, from the landlord.

I didn't think it was legal to lock them out.

Montana is freezing, there are children involved. They have absolutely nowhere to go. And all of their things are inside the house.

The kids don't even have their clothes for school!

That is absolutely illegal. They should call the police and report the landlord.

@Jess Kinzel -  In NY it's super illegal.  I would assume its the same everywhere else.  That's a lawsuit waiting to happen

This is illegal in Montana.  The tenants should call the police, who will get them back into the unit.  The LL has to go to court to evict them.

This is absolutely illegal.  Contact the police immediately.  They have to be evicted legally through the courts.  He can't just make up his own rules for a little late payment.  If they wanted to they probably could sue him.

This is known as "self help" and as far as I know is illegal in all 50 states. Eviction is a judicial/legal process. My guess is that this wasn't the first time they were late on rent and the landlord let his frustration cloud his/her judgement.
Also, you may not have the entire story. There may be more to it than what was conveyed.

As all have said before me, this is an illegal act.  And, downright shameful.  

What I would offer additionally is that tenants need to seek housing assistance BEFORE their rent is due.  And, don't stop seeking assistance while you're calling the police and moving back in because the landlord will be starting eviction proceedings. Pay partial rent NOW if the landlord will accept it and let him know when the remaining will be paid - and let him/her know that you're seeking a more permanent resolution so this doesn't happen again.  You cannot over-communicate nor can you put off seeking assistance.  

When you don't pay the rent, you're stealing... plain and simple. The landlord has mortgage payments, insurance, taxes, maintenance/repair expenses so he/she is the greater stakeholder.

I'm sorry this happened to you.  Please do what you have to do to ensure that you can pay your rent in full and on time going forward.  

It is illegal, no matter how many times they've been late or how long they are late.

Originally posted by @Jess Kinzel:

In the state of Montana , a renter fell behind on rent due to job loss.

The landlord sent them a letter saying they had 3 days to move out.

The tenants were trying to get help through some government agency..

On the 5th day the landlord changed the locks and locked the tenants out of the place while they were gone.

Is this legal to do?

Absolutely illegal and reprehensible. I hope whoever is involved here brings the HAMMER down on this scumbag. Nothing makes me angrier than someone who is pretending to be a landlord but does not treat the responsibility with the respect it deserves.

The person who was kicked out should absolutely be able to call the police and file a report. If they have proof of address, the cops should even be willing to help persuade the owner to let them in. (Not 100% sure on that part as it may be state specific.)
Their next step should be to secure legal counsel. Some municipalities offer free/reduced price legal aid for tenants. That can be a good first step. Since this could be a slam dunk case, it may even be possible to find a good attorney to do it on retainer.

I hope the owner in this situation loses their house over this.

Obviously not legal, but can we talk about the stones on this landlord for a second? Wow. Don't tell me as a landlord you have never wished you could do this while in a fit of rage. To actually do it is a whole different level. 

Just one question.  Was the 'tenant' working for the landlord?  If so that might change the answer about whether it is illegal since housing that is supplied to an employee ends when the job does.

Originally posted by @Kathy Johnson :

Just one question.  Was the 'tenant' working for the landlord?  If so that might change the answer about whether it is illegal since housing that is supplied to an employee ends when the job does.

 Nope. No work relation at all. Just a renter and a landlord.

Assuming this is a long term rental they have been in more then 30 days,if the rent is 3 days late or even more then that but there is no eviction process through the courts  yes it is illegal.  Tell them to call the local legal aid in addition to police.  

Obviously this is illegal based on the limited one sided information you gave ...however there’s two sides to every story . I’d like to know more about the tenants not paying rent and why they are a week into the and only then seeking assistance . What’s the need for the assistance ? We’re they fired ? We’re drugs involved or criminal activity ? Do they work ? What’s the backstory here . Sounds like if they paid the bills they wouldn’t be in this mess . Just sayin 

Michael, I am totally with you in your admiration.  A friend of mine used to LL in New Britain and Hartford.  He never took them to court.  He threw their stuff out on the street, and he had some really scary muscle to do it.  Nicest guy in the world.  I don't have the nerve to do it.  I go to court, even though I wind up losing money on every legal eviction.

In Texas, it is not illegal to lock out a tenant.  However, the landlord must be ready, willing and able to provide the tenant a new key and access when requested

So while you can, there is no point