Threesome with my girlfriend and best friend?

Ok my gf and and bff talked about having a three some together. We are all girls and all lesbians. We also talked about having an open relationship with only my friend. We are all very close and love each other. Do think this is a bad idea and will tare us apart or do u think it will bring us closer together? P.S i don't want anyone saying "being gay is wrong you'll go to hell' its my life