Can I convince my girlfriend to try anal sex or a threesome with another girl?

I have been happily dating my girlfriend for about 2 years now. Although the sex is great I want to try some anal sex or a threesome with her and another girl. She currently isn't open to trying either, can I convince her to try one or both? Are there detrimental effects that can result from having a threesome? Or am I just selfish for wanting to try a threesome or anal sex?