Early gameplay in World of Tanks
puts you in Light Tanks. You're generally required to work your way through the tech tree, advancing your tank technology to a point where you can eventually research and then
buy Medium and Heavy tanks. The tanks in Founder's Pack #3
will set you up with two Mediums and a Heavy Tank from the get-go, without eating up your existing Garage Slots.
To me, the big win here is the KV-220 Heavy Tank, because, well, ...Heavy Tank. The other two tanks are Medium Tanks and you can access Mediums quicker than you can access Heavies. The KV-220 gives you an early chance to try out the much slower, but much, much harder hitting world of Heavy Tanks.
The other two tanks seem to have been chosen to give some balancing factors that will likely make some people disagree with my favoring the KV-220. Specifically, both the T-34-88 and the T23E3 are higher tiers than the KV-220, giving you a total of one Tier 5 Heavy Tank and a Tier 6 and a Tier 7 Medium Tank. Further, while the American T23E3 is the Tier 7 tank, the T-34-88 has its own unique feature that makes it attractive...