I think you should talk to them, but maybe approach it differently than just calling out the pattern. Before you do though - have you noticed if there's anything specific happening in their life right now? Like, are they going through a rough patch, new job stress, relationship stuff? Sometimes people who cancel a lot aren't actually losing interest; they're just overwhelmed and don't know how to say it without feeling like they're letting you down.
When you bring it up, frame it around wanting to understand rather than accusing them. Something like "hey, I've noticed plans keep falling through and I'm wondering what's going on? I miss hanging out but I also don't want to keep making plans if the timing's not working for you right now." This gives them space to be honest about whether it's actually about them being flaky or something deeper. You might find out they're anxious about canceling, or stressed, or going through something they haven't told you about yet.
That said, if they do know the pattern bothers you and just keep doing it anyway without any real explanation or effort to change? Then yeah, you can pull back on inviting them as often. You're allowed to protect your own energy. But I'd give the conversation a real shot first, because three months is long enough that this probably matters to you and deserves to be addressed directly.