That's what I tried from the start after reading the forums about the M3 Lee. Problem is, this damned "tank" gets matched against better tanks far more often than not. A TD with that tiny gun is useless and I can only take 1-2 hits from just about anything. I guess this is just an OK tank/TD that is generally outclassed by anything it gets grouped with.
Now...the M5 Stuart seems like an amazing tank but the matchmaker seems to hate it. In the rare game where I'm at least halfway up the list, its a fun tank to play and it can hold its own with the derp gun. Sadly, about 80% of the games in the M5 are games with me at the very bottom of the list. In those games I'm generally better off just hiding somewhere and spotting. Eventually (almost always) a heavy finds me and he knows that I can't outrun him, outgun him, or even outturn him! Basically, being spotted means almost certain death. I'm nearly useless in the M5 and the amount of XP per battle is extremely low.
Ah well that's just a welcome to tier 4, not so much a problem with the M3 Lee or Stuart. I think the Stuart actually get boned more than the Lee with the matchmaker because it's a light tank.
At tier 3, the highest tier you will see in a match is a tier 5, essentially a two tier spread. Starting at tier 4, it goes to a three point spread. So you can see tier 7 tanks (Tigers, T29s and IS) with your M3 Lee. If you're thinking, 'there's not a goddamn thing I can do against those tanks,' you're absolutely right. You need to drastically alter your approach to the game starting at tier 4.
When the game starts, you need to determine whether you're a big fish or a small one. One complaint that a lot of people have is that small fish have no roles. That's not true. People are really complaining that the small fish can't run around killing everything they see, and that's totally true. You need to expand your game to roles where killing things isn't necessarily on the menu. Scouting intelligently (move towards the line, hide, don't fire, dash away when found, repeat), wait-and-dash (where you wait until everyone's engaged, find the hole, and then make a break for it to bad the arty), gun support (sit behind the fray and lob HE shells at everyone bad), or fire man (use your speed if you have it to go where the current problem is) are all things low tier tanks can do in a high tier game.
I think, honestly, that figuring out how to play low-tier in high-tier matches is what separates the men from boys and the players that stay with the game from those who quit.