As far as I'm concerned, and I've participated in quite a few threesomes, you wouldn't ignore your partner during sex in favor of masturbating unless they were clearly into watching you masturbate -- that would just be rude -- and, similarly, you shouldn't let anyone feel left out and on the sidelines during threesome sex unless they have chosen to step back for a bit and/or are clearly excited about watching.
Over time, you may gain a sense of familiarity and ease with two other people where things can be a little unbalanced one session because, it's cool, they paid a lot of attention to you last session and who's keeping track? But it takes a while to get there. For the first several times especially, each partner needs to work to actively include the others. This does, in fact, take more awareness and thought than some people may be used to expending when they're busy getting down, so I'm not surprised that some folks get it wrong at first. But if they're not working to include everyone they ARE wrong, in that the person being ignored ends up feeling bad AND the person being focused on will probably end up feeling bad too since no one wants their pleasure to come at another's expense.
There is very much a time for intense, bonding, stare-into-your-eyes, give you all the orgasms in the world, feeling like no one exists but you and me, dyad-focused sex and, hey, guess what, it's when it's just the two of you. That's why it's so foolish, imho, when people think they can form authentic, lasting sexual/romantic relationships without ever having *any* dyad time. A triad is three dyads PLUS the triad.
To the OP, your husband's mistake was just that, a mistake born of ignorance of how these things work. I'm sure it was in no way meant as a slight, but he needs to recognize why his actions were hurtful. And why on earth is he mad at you, because you asked them to stop? If so, that is just selfishness, you have nothing to feel guilty for as you were just as much a participant as them in that sexual encounter and, therefore, if you need it to end it ends. End of story.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.