I am a newly single mom of 3 looking for a newer model convertible that has 3 seatbelts in the back seat. Something I can fit a car seat and 2 booster seats.
Thanks!
A Mini van with a sun roof 
http://cgi.ebay.com/ebaymotors/1959-Cadillac-Eldorado-Biarritz-Convertible-Tri-Power_W0QQitemZ120215925173QQcmdZViewItem?hash=item120215925173
http://cgi.ebay.com/ebaymotors/1965-Cadillac-Eldorado-Convertible-One-of-2125_W0QQitemZ200199515148QQcmdZViewItem?hash=item200199515148
http://cgi.ebay.com/ebaymotors/1966-CADILLAC-COUPE-DEVILLE-CONVERTIBLE-RESTORED_W0QQitemZ360021899160QQcmdZViewItem?hash=item360021899160
If you can’t fit 3 car seats in the back, I have to wonder just how big they are.
Unfortunately, no newer models exist with the room you require.
The new Jeep Wrangler unlimited is the only model I can think of that has three seat belts. Three car seats is probably going to be very tight.
The truth is trying to safely fit 3 child restraints across nearly 95% of normal four door cars is quite difficult. Only a few wider cars can accommodate this, minivans and large SUV’s.
Convertible actually shrink the width of the rear seat since the convertible mechanism narrows the available space. I think a Wrangler Unlimited may be the singleton choice that is modern.
Forget it. The roof mechanism takes away shoulder room and there is only enough room for 2 in the back, even on huge cars like the Bentley Azure and Rolls Royce Drophead Coupe.
I hope that the circumstances that have led to you being newly single were not tragic. If they were, my condolences.
If you really have five passengers, I think I see a minivan in your future.
If you really have your heart set on a convertible, why not get the passenger air bag disengaged and let one of them ride shotgun? It would be better than this!
Why not a Mustang? I assume that there are three belts in the back, but I don’t know if your hardware will fit.