Short answer…yes.
Long answer…at this stage, I would recommend against that. This game is a grind and to compete in wars, you need your top 30 heroes. Based on your screen shot, you don’t even have 30 heroes. I’d instead recommend buying a few extra team slots so you can keep more - they are only 50 gems for a while. There are other threads to offer advice on leveling, or you can ask here. But yes, as you advance, you will use 3* (and even 4*) as feeders but I wouldn’t do it now.
My thought would be to hang onto and eventually level at one of each 3*. Not that I’ve managed to do that yet, even after over a year playing. 
Here are some reasons:
- It’s worth hanging onto them at least until you have a set of 30 or more better heroes, for alliance wars.
- The easiest levels of monthly challenge events also restrict you to 3*, so they may still be useful there later on.
- The class events that are now twice per week restrict which hero classes can be used, so you may find yourself using them there.
- And there’s some sort of new feature in the works with variant restrictions on heroes that include the chance of using no better than 3* to compete.
Using 3* heroes as feeders isn’t the best choice for new players:
you could need up to 30 of them to have good results on alliance wars and you could have troubles by not having heroes to use on the newly added class-restricted missions.
You are correct, I feed all * and **. And do what I can in alliance wars, I usually manage to knock out 2-3 and the occasional 4th opponent.