Members of a three-piece band look back on a show, and one complains about how Mishima's final chorus went.
The words appear when the audience laughs.
でも、お前文句ばっか言うけどさ、今この三人のこの形があっても、
But look, you do nothing but complain, but even with the three of us in this lineup right now...
Textbookでも、あなたは文句ばかり言いますが、今この三人のこの形があっても……Textbook 'ばかり' gets an emphatic small 'っ' ('ばっかり'), then clips further to 'ばっか': '文句ばっか言う' = 'always complaining'.