3* Heroes come in steady supply once you advance in the game, 4* will form the backbone of your roster.
As a F2P myself learning this game as I go (forum helps tremendous, as you would know), you will soon find out that there is going to be long road with a lot of waiting for upgrades to finish on your way to TC20, because not only will you need to shoot your Stronghold all the way up to 20, you need your Iron Storage of all 5 of them to be at least Level 18, Food Storage up (I don’t have the numbers, but I did it with 13, 12, and 11x2) in order for you to completely research the Legendary training, your housing situation needs to be on par as well. There are going to be times when you are waiting for one building to finish, but have hoarded enough food / recruits that you are about to be full and cannot collect any more, but your food from your farms/watch tower are still trickling in. So I needed place to spend them, I always end up putting them in TC13 because it can help me burn through some of those food and recruits.
In the times I’ve spent waiting for the stronghold and support buildings, I managed to get some (not a lot) of 4* that I could use, and still plenty of 3* came in. It might be just personal preference, but I do not like to see production buildings (farm, iron, watch tower) with yellow (full) collection icon and messages telling me that I don’t have enough recruit housing space. So whenever I get the chance to spend them, I do. Besides, there is also one area aside from the food/recruit cost that you need to take into account, is that they both take 2 days to finish one training session. Typically, with active playing, you should be able to get enough food and recruits to start another session of TC13.
Another thing I would suggest is not to do too many building goals at once, try to focus on one at a time(stronghold) and any support buildings needed to get it up there as needed. If you try to evenly level things out, you will get there, but it might take you longer time to start getting viable heroes. If you have VIP Pass, the second builder can help alleviate the wait time. I never purchased the VIP Pass, so there were a lot of waiting of buildings to finish.