I don't think getting 3-stars has any influence on the chance of winning better gear.

In terms of experience, the less champions used, the more experience each champion will gain.

experience is equally spilt between everyone. if your leader is  max always use a full team to not waste xp.

4champs all get 25* so only 25% gets wasted on your lead.

if you have a max lead and take 2 with u. they'll get more xp but the lead also. 33% per champ

so if you take only 1 u waste 50% of the xp.