We are planning on combining Jake's, Riley's and Owen's birthday parties this year since they are all within about 2 weeks of each other. I am inviting some people for just one kid, then others for all 3 kids (like grandparents) When I mentioned this to someone, they said I should at least warn people the party will also be for the others. I'm afraid that will make people feel obligated to bring gifts for all 3, and I absolutely don't want that to happen. However, the person I was talking to said she would want to know if there was going to be more than one birthday child, even if a gift was not expected.

So, when doing invitations, should I put something in them about the other 2 parties?

I would go with something like this...
Come join us for a birthday celebration!
Riley/Owen/Jake would like you to come and be part of her/his/their party.
or
Together with her brothers, Riley would like you to come to a birthday celebration.
Together with his sister and brother....