That depends on the raidboss. For Alakazam you would need pretty much all maxed out Tyranitars but for Machamp a team of 2 lvl Alakazam, 2 lvl 30 Espeons, 2 lvl 30 Dragonites (hurricane) would do. It also depends on the moveset of the raidboss.
In general, for Vaporeon and Machamp a team of optimal counters at level 30 would do. In order to defeat other lvl 3 raidbosses you need to power the optimal counters past level 30.
Goodluck!