Hi Ms. Realgirl,
Regarding:
The guy [my lover-in-waiting] is hot but a total player; I don't think I would want to have an actual relationship with him. But, I have been flirting with him. I'm not sure how to bring up a three-way with my BF.
Flirting with your lover-in-waiting (whom you don't want a "relationship" with) is the right way to let him know you want him sexually, so you're on the right track with that. Since you're the one who knows your BF (whom you want to retain as such, while accepting you having a lover), I think you'll have to be the one who decides how to guide him to agreeing to a threesome.
One way might be, during your next make-out session or when you're in bed with your BF, start giving him a handjob and, as you do so, begin "talking dirty" to him. (Men are turned on by that.) Then, when he's hard and starting to approach orgasm, suggest the possibility of a threesome. ("What would you think of the possibility of a threesome, baby...? Have you always wanted to watch another man do me...?") Then, stop your handjob while you look into his eyes, awaiting his answer.
If your BF responds in a positive way (says "Yes, Ms. Realgirl, I would..." or something to that effect), reward him by resuming your handjob and bringing him to orgasm. Then, whisper in his ear saying things like, "You liked that idea, didn't you, baby...? Hmmm...? I think you WOULD find that hot, wouldn't you, sweetheart...?" But, don't worry too much about the nature of his response after he cums, at which time he might be (or appear to be) less interested in the idea of a threesome.
If, on the other hand, your BF responds in a negative way as he's approaching orgasm from your handjob (for example, he hesitates and doesn't reply... or says something that amounts to an expression of reluctance), then don't reward him. Rather, stop your handjob and say something like, "That's enough for tonight, baby... I don't feel like it anymore..." and bring your make-out session with him to a close. If you're in bed together, for example, roll over and go to sleep, or pretend to go to sleep, leaving him with the choice of having to play with himself or remain frustrated.
If your BF's response seems sort of negative but he does not strongly reject the idea of a threesome, I suggest continuing to work on him over time using this handjob strategy. Don't give him blowjobs, because it will be essential for you to talk with him as you fondle his cock and especially as he's approaching orgasm. That's when he's most likely to agree to what you want from him, as you pause your handjob and look into his eyes, awaiting his response.
Each time you get a positive response that suggests your BF is moving closer to agreeing to what you want, I suggest rewarding him by resuming your handjob and bringing him to orgasm. Each time his response is negative and isn't what you want, I suggest declining to reward him by stopping your handjob and terminating your make-out session, as above.
Throughout, it will be important to do all this... should you decide to proceed along these lines... lovingly and seductively. Don't treat your BF harshly, because that could result in losing him (which you don't want, it sounds like).
—Custer