OK Iv finally figured it out it depends on how advanced your costumes are. Like when you complete stage 3, costumes in stage 3 will have low stamina, but when you apply any amount of costumes you've acquired in stage 4 with a costume in stage 3 (like if you have 3 pieces of costume from stage 3 and 1 piece from stage 4) then your stamina will greatly increase.
So the same thing applies with stage 3, the more costume pieces you acquire in stage 3 and add t to any costume in stage 2, your stamina will increase! I hope that made sense
Anyway, how I completed stage 3 is that I would start in Stage 2 and collect as much milk from that stage as possible so that I would have a little more milk on stage 3. The way how you could continuously have not very little milk on stage 3 is to have a kitty dressed up as one of the South Park character costumes when you are able to acquire one ;)