The grass is always greener on the other side of the fence. Threesomes and relationships are a big difference, as I'm sure you know. Make sure this is something you really want and not just a fantasy before you open those flood gates, because once you say it/act on it, you cannot go back.
A good way of bringing this up to your husband, if you haven't already, is to say, "We've talked about threesomes in the past, and I'm not big on casual sex. I feel like you're interested in having sexual relationships with more than one partner. Would you be interested in dating another girl while staying married to me?" Put it in such a way that you're not saying, "This is what I want" but instead saying, "Would you be interested in this?" so that he's not on the defensive.
If he says hell yes, say, "The only way this works is if we are both allowed fair opportunities. I would like to add someone else as well, and if that's not okay for you, I need to know. If it does, we need to go over ground rules and talk about this again in a week or so to make sure we're still on the same page."
Then discuss it for a month or so every couple of days to make sure feelings haven't changed.
Only after you've gone through all the conversations you need to go through should you even consider looking. Otherwise, heartbreak.
"Regret for the things we did can be tempered by time; it is the regret for the things we did not do that is inconsolable." - Sydney Smith
Kyle: 26 year old male
Katie (rymmare): 24 year old female
Kids: girl: 4 years old, boy: 2 years old