These three Motley Fool contributors think now could be the perfect time for biotech investors to add Spark Therapeutics (NASDAQ:ONCE)  Esperion Therapeutics (NASDAQ:ESPR)  andCelgene Corp (NASDAQ:CELG) to portfolios Why? An overreaction to trial data could mean Spark’s shares are on sale  Esperion Therapeutics has big news fast approaching  and Celgene may be one of biotech’s best bargain-bin buys