well you don't exactly HAVE to fight those two.
when you kill a boss, it takes you back to the wierd dimension with the stairs right? then it shows a block with colours in, and a colour lights up after every boss, right? the way to end the level is by lighting up any colours on the block that are in a joined circle. then a portal should appear that ends the level. the only reason it's telling you to kill those 3 bosses is because that is the smallest circle on the block. kill all the bosses and you get a blue orb shard, that's all.
did this help?