"How it worked in the last weeks was that they've been seeing each other every now and then"
I don't consider 2-3 nights a week to be now and then either, that can be a struggle for couples no matter what. Have you talked to her about your feelings to see if she really wants to see him that often regardless, or if she just doesn't know what your feelings are about it? I tend to be of the opinion that starting slow is good, and making sure you're being considerate by checking in on how things are going for your partner is extra good, is she doing this for you? Are you telling her honestly how it's going?
I have only been in poly relationships while married, but even my most serious long term other relationships took up 1-2 nights a week. Unless I was single and dating other "single-ish" people I don't imagine many people in long term relationships manage to be available more often than that. My husband and I have negotiated the following.
1 date night OUT OF THE HOUSE for us a week to cultivate our connection, without chores or computers to interfere.
1 set day a week where we are home to discuss any issues that have come up between us or in regards to our other relationships.
4 nights apart max a week on dates/hobbies/events, whatever
No more than 2 nights a week on dates with the same person unless we want to negotiate otherwise (which would only happen if we both felt really damn happy and content with the amount and quality of time we were spending together).
This is what works for us at this time, and it did take quite a bit of thought and discussion to figure it out, and some false starts. And it IS good of course to be finding hobbies and friends to keep us busy in case some day finds one of us single (I meant married, just...not dating others) & needing to fill 4 nights a week so we aren't sitting at home lonely and resentful, but Rome wasn't built in a day, so give yourself some slack.
I didn't get if you live together or not, but if all the fun is with him, and you're stuck doing chores and errands and whatnot together besides the family stuff, you probably want to talk about that sooner than later. Scheduling enough opportunities so we have free time for us and love and sex after all the domestic crap is done is the only thing that's going to keep us both happy at least. Don't be afraid to us those I statements that MorningTwilight mentioned.
"I am glad you are having such a great time and I like seeing you happy, but I realized I am feeling like we aren't spending quality time together as much anymore, and I would like to see if we can make a schedule so everybody is getting their needs met" or whatever it is you think you really need to ask to make it so you're happy too. Don't be scared to say that you feel you're missing sex too, but maybe if you start negotiating without bringing that up you can get the changes you like without shining the spotlight on that. I really have seen the difference in outcomes between the people who speak up early on vs. the ones trying to keep their feelings to themselves because they don't want to seem "needy" or upset their partner by having the nerve to have feelings! It's those former ones who have more success btw