The rule for nesting rides is you can't nest a ride if you had to have done one to do the other.
So a BBG, 1500 miles in 24 hours is not necessary to do a B2B Gold ride. So you could nest a B2B Gold inside a BBG.
1-15 E2E is slightly less than 1500 miles. So again, you would not have to ride a BBG to complete a I-15 E2E ride. So you could nest a I-15 E2E inside a BBG ride.
An I-15 E2E does not go to the Mexican border. So you wouldn't have to do a B2B ride to complete an I-15 E2E. So you could nest a I-15 E2E inside a B2B ride, inside a BBG ride.
As I understand the guidelines for nesting, (and it's really up to the IBA what they will allow), you could ride a BBG that also encompassed a B2B Gold and a I-15 E2E ride, but you would need to document each ride's start and end points. That means a DBR to start your BBG, then receipts across the border, (not sure if you could start the BBG deeper in Canada or Mexico w/o it becoming a metric ride, but probably ok), then the start and end points of the I-15. Since the US/Canada border is one end of I-15, the B2B receipt in Coutts might qualify for that end of I-15 too. Or you might get a DBR in Sweet Grass just to make sure.
Hope that helps.
@Ira will chime in if I've gotten it wrong.
Do understand that while you would potentially get 3 certificates, for the purposes of a Mile Eater cert, all three certs only count as one ride.