Top 3 should be the rule but like others said give others a chance to go up, something to fight for and well £'s.
Also, the top 3 likely are clubs with more resources from epl so might be hard to crack the top 3 ( like it is for us to crack the top 6 in the epl.)
Heck, maybe epl
/uefa should do playoffs for 2 to 8 on who gets champions league vs europa vs europa.conference to give others a chance? Lol not happening huh?