You could say,
I did a 3-month internship at Company A.
I am currently doing an internship with Company B.
Then perhaps add, that you may or may not be hired on full-time at Company B, depending on how Company B's startup offering fares in the market.
Then, be prepared to discuss what you learned during your internship at Company A, what skills you acquired, what contacts you made, in a positive way. And, of course, the same for your experiences at Company B.
Ultimately, it is up to you to be selective in choosing which of these "internships" to take on, and choose the ones that will best prepare you to be appealing to employers who will pay you. What in-demand skills will this internship provide you?