You do not have to actually buy the Tier 3 tank, you just have to research it...
The absolutely cheapest Tier 3 tank in terms of XP needed to buy is I believe the Soviet Light Tank T-70, down the tree from the Tier 2 T-60 and the Tier 1 MS-1.
The thing to remember, if you're going for this really quickly, there are a lot of tanks which require you to buy packages on that tank before you can research the next tank. This means that for some of the Tier 3 tanks, you need up to 3,000 XP before you end up being able to research the Tier 3!
The Soviet T-60 has only one package to research before you can skip the other packages and go straight to researching the Tier 3 T-70.