Penis size debate on this issue http://penissizedebate.com/page29_great-equalizer.htm It explains everything about flaccid to erect.
To quickly sum it up. Would a 3” flaccid have the same erect length as a 6” flaccid? The answer is obviously no. Odds are when the 3” flaccid becomes erect it won’t even be as big as the 6” flaccid.
Just as an example my 3” flaccid penis grows 166% to get a NPEL of 4.9 something. A 6” flaccid grows 134% to become 8” Both grow two inches from flaccid to erect.
Start 5.25 BPEL x 4.36 EG
Current 6.2 BPEL x 4.60 EG