A.C.
At this age, 3 months is such a huge difference, but as the kids grow, that gap will become less and less apparent. Our play group had babies from January to June. Some were mobile while others couldn't get off their backs, and we made it work.
It sounds like this gal was offended and this is easy to fix. The next time you talk to her maybe just say something along the lines of...the last time we talked I got the feeling you were upset with my feedback - my intentions were not to hurt your feelings and I didn't mean to upset you. I'm really sorry. I'd really like to keep coming to this group and I came up with some suggestions for meeting places that might work for everyone.
If the group alternates places to meet that work well for you, then you can still continue to go by either compromising, or just attending the locations that work.
It just sounds like she put the meeting together and hearing that you didn't like it may have made her unintentionally feel bad, so if you apologize, offer some suggestions and if she's a reasonable person, it will be fine.
If you like the kids and the other moms, try to stay in it...it's good for you and good for your son.
If they continue to meet in places that don't work for you, or it turns out she's not a reasonable person, then maybe contact the moms with the older babies and see if they would be interested in getting together another time during the week.
Good luck!