I have been quite happy with
Choice Air
but you do have to speak to them and specify you want to use the Onsite company, as there cheapest prices are for the offsite ones, and its just not worth the hassle.
We generally get an Economy or Compact, and have had a Dodge Neon or equivalent which was quite comfortable for 2 adults and 1 child in a car seat.
For 3 adults you might want to look at an intermediate. I would definitely recommend 4 doors.
I think for 2 weeks with all insurance included and an additional driver and a tank of petrol, we have been quoted £330 for next June.
You can also try
Florida Car Hire
or
Bon-Voyage
Which airport are you flying into?
On Airport at MCO is
Avis, Budget, Dollar, L&M, and National
Off Airport (meaning a shuttle bus ride is needed) there are tons more
Click here for the list
at Sanford
On Airport is Avis, Alamo, Dollar and Hertz
Off Airport, Budget and Enterprise.
Hope that helps,
Bev