I have a huge lump under my armpit when it came up i thought it was a boil and it has tripled in size in the past 3 days very painful and hurts ?

Abscess : Sounds like the blister has turned into a subq abscess and cellulitis .. You may benefit from a quick ER or urgent care visit for evaluations , they maybe able to do a quick I & D and send you home on oral antibiotics ...

Need an exam: It sounds like an infected cyst or an abscess. In any case, you need to see your physician today. You may well have a problem that needs to be surgically drained or excised.

See a doctor soon: A new lump in the armpit should be investigated by a doctor without delay. Possible causes include infection and cancer. It cannot be diagnosed via internet.