When it comes to 3700 series APs they are first supported in 7.6 software version.
For this piece of hardware we really do not see any restrictions when it comes to the flexconnect feature on the config guide for 7.6
Also, this devices are new on this software version since they are implementing new features like AC radios in addition to all features previously supported for APs working in centralized unified wireless networks like flexconnect.
Restrictions in Flexconnect are not really limited to hardware on the APs but mostly to other network characteristics when it comes to implementing this type of the deployment, here you have a list:
Hope this info works for you.
Just to add... They are only supported in AireOS controllers, not in converged access.
Yes We have 3702 AP running in FlexConnect mode. You also can go through the below link, it has a complete list of APs and Controllers which will support Flex connect in our cisco products.