4 extra copies
Average to Min-cost: 7
CR Chances:
1st: 100%
2nd: 50%
3rd: 25%
Ignoring the 1st (since it's guaranteed), getting the 2nd and 3rd in 3 tries and assuming it took 3 tries and not 2 (you did not have a spare copy left over):
.5*.75*.25 = .09375
.5*.5*.25 = .0625
Add the two and you get .15625 = 15.625% chance of it happening, slightly better than 1 in 7.
Yeah, you were lucky.