Help in determining value of Triplex

[Wayne Gilroy]

I have a triplex that I've owned for almost 25 years.  I'm trying to determine it's value should I decide to sell.  Finding comps in this area is difficult and the market seems to change a lot.   I have talked to a couple of realtors, but I feel they are pricing it kind of low.

I'll try to be brief and concise with the property description and numbers.  I would appreciate any feedback and opinions.

Age ~ 1940s

Unit 1: 450 sq ft. kitchenette. 1 bedroom. Full bath. Fairly up to date. Current rent 650.00
Unit 2: 800 sq ft. 2 bedroom.  Onsite laundry (shared). Recently updated.  Current rent 750.00
Unit 3: 800 sq ft. 2 bedroom.  Onsite laundry (shared). Attached 1 car garage. Needs updating but tenant will probably stay there until he dies (his words!).  heavy smoker and no reason to update until he moves out.  Current rent - 700.00
Detached garage - Less than 5 years old.  Interior is separated down the middle.  Also a separate entry and area just for landlord use.
Each garage unit current rent is $50.00

Total monthly income - $2200.00 x 12 = $26400 annually

Utilities - Owner paid as nothing is separated.  2016 for water, gas, electric - $3600
Storm water utility bill - $500.00
Insurance - $1000.00
Taxes - $3300.00
Misc Maint. $500.00 (lawn care, salt, paint, minor repairs)

Total annual costs - $8900.00

So roughly 17500.00 per year net income.

I do most minor repairs and some lawn care.  No management company.

The neighborhood is desirable.  Good schools, a lot of dog walkers and joggers in the evenings, quite streets, working or retired class type of neighborhood. The town is small enough to only have one large high school, but several elementary schools.  About 30k city population. I never have a problem renting any of the units.

The property will need a new boiler at some point (plumber quote of ~6k).  Electrical could use some updating.  Probably Switch two electric stoves and electric dryer to natural gas at some point.  
Building has nice yard space, siding less than 5 years old, roof about 10 years old. All unites have off street parking.

County tax assessment is at 150k for the property.  Realtors' seem to think it should be listed for 130k - mostly due to the 3rd unit needing all new paint and flooring when the tenant moves out or dies.

Anyway, I'm looking for opinions on how to better assess this income producing property's value.