cluster with 3 appliances

This is certainly possible, to do this you will use device and traffic groups. Device Groups to define the cluster members, and traffic groups that define the services that are active on which cluster member.

Active-active config https://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/bigip-device-service-clustering-admin-11-5-0/3.html

Device Groups https://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/bigip-device-service-clustering-admin-11-5-0/6.html

Traffic Groups https://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/tmos-redundant-systems-config-11-2-0/6.html

Hi A_Azeddine,

You can have a device group of 3 devices.

Its a good approach if you run multiple traffic groups in a active-active mode. In this case unit-1 can host traffic group 1, unit-2 can host traffic group 2 and unit-3 can be used as a failover device for either traffic group 1 or 2.

if yes, what are the requirements?

You have to use the same plattform as well as same TMOS version and you have to use Network based failover (the serial-cable won't work for 3 units).

Cheers, Kai

You have to use the same plattform as well as same TMOS version

This article seems to indicate otherwise about platform compliance, with restrictions of course.

https://support.f5.com/csp/article/K8665