Every pu of a 3/4/5star character you have a chance to get a bonus hero of the same level. If you don’t select a favorite for a tier, 3star in your case, you will get a random 3star when you would pull a Bonus Hero.
What you have done does nothing to improve the chances of getting a 4* bonus hero. You have only ensured that you will get a random cover when a 3* BH is pulled. If you don’t care which 3* covers come your way, then this is fine.
I personally prefer to focus on one 3* hero at a time so I have one 3*, one 4*, and one 5* selected. Once I hit a point where all of the 3* are champed (or at least all of the ones I care about), I will choose my primary 3 and leave it until they approach max level.