Hi @Daniel Oren
GC's get their subs the same way everyone else does. Networking, referrals, trial and error. Often they can get better prices because they have repeat business and they know what a "good" price is from doing it all the time. You don't need to be a GC to hire "sub"contractors but you need to be aware of your subs licensing and insurance. Additionally you would need to pull any building permits that were needed. Its not unusual for investors to GC their own rehabs (although not advised when you scale up) but be aware that your "subs" can take advantage of your being unsure of going rates. Take your time and get multiple sub bids on at least one of your flips so you have a greater ability to judge the going rate on your future flips.
Additionally, if you do choose to use a GC, make sure you have a timeline with milestones in place up front. This is worked up with your contractor, not something that you come up with on your own and hand to him. That way, you have a written expectation of what needs to be done and when and if a milestone isn't met, then you can evaluate whether it was reasonable or not. I've heard of people providing incentives for finishing early, or penalties for finishing late. As for your current guy, you may just want to sit down and express your concerns and map out your timeline going forward. If you aren't on the same page, that makes it hard for him to meet your expectations and hard for you to know if your expectations are reasonable.
Its a good idea to have more than one contractor for any given service (sometimes easier said than done. At least in my market... ) Sometimes one isn't available when you need him and you need to have another and if you only use one all the time, you are likely to get price creep.
Congrats on having several flips going at once and good luck!
Lots to unpack there, but first of all Congrats on working 3 deals at once. That shows hustle!
I would try to find references from other local Real estate investors. Do you have a local REIA? I have used Thumbtack and Home Advisor with mixed results.
I would not use the same GC for all three deals, especially if you do not already have a good working relationship with them and know the quality and speed of their work. What if they decide to up and quit on you or you need to fire them? If they were working on all 3 projects then all productivity stops for weeks until someone can (hopefully) come in an pick up the slack