I bought the Tasca Sherman II and is going to build into a nice kit. After doing some research the American used mostly the M4A2 which the British called Sherman III. Tasca just released one, what are major diffrences between the III and A2?

In terms of the basic vehicles? Nothing. The only differences are in the stowage and radios. The British added exterior fire extinguishers and (maybe) a couple of British pattern pioneer tools in addition to the ones shipped with the tank from the US. Depending upon when it was in service and exactly which turret it was fitted with it might have the external smoke grenade launchers and maybe a Brit pattern external stowage box behind the turret bustle.

Aerials would be for the No. 19 set.

Individual tanks in specific theatres also had other fittings added, but that is really specific to the unit, location & date.

HTH

Paul

Hang on, the US Army used the gasoline-powered M4 and M4A1 in combat in the opening campaigns of the war, not the diesel M4A2. The M4A2 was mainly reserved for Lend Lease users, notably the UK and the Soviet Union. The only American users of the M4A2 were the US Marines, who favored the diesel variant until nearly the end of the war (on Okinawa, all but one Marine battalion had switched to the M4A3).
The M4A4 Sherman V, with its distinctive stretched hull, was also used extensively by the British.
The US Army used the M4A3 and M4A4 for training in the continental United States in 1942-43. In 1944, the Army began to favor the Ford engined M4A3 variant , and from August, 1944, the M4A3 became the predominant type in Northwest Europe (though these were nearly all late production variants with the 47 degree hull front).
The direct vision Tasca Sherman III kit is suitable for a British Lend Lease version, but the USMC tanks were later types without direct vision slits (the Dragon M4A2 Tarawa and Sicily Sherman III kits depict this version). The Tasca Sherman III could be converted to an early M4 with a replacement resin engine deck and new tail plate and exhausts (and a new hull bottom, if you're a stickler, since the access plates were arranged differently on the bottom).
But it seems certain that an early direct vision M4 will appear from Tasca or Dragon very soon. For now, though Dragon's Normandy M4 is the most versatile M4 kit currently available, as it has alternate parts that allow the construction of a host of 1942-43 production versions, and could depict a tank from the Italian campaign through the end of the war..