I think it was mentioned by @farmboy to award 5 points to wins against tier 3. I think that is a good suggestion. Also you could remove the 3 game cap, so you have a rating after 1 game, it also means you would earn 5 points if you beat an unranked player:
A win against an unranked, tier 3 or tier 2 is awarded with 5 points. By removing the 3 game rule before you are ranked you also make it easier for new players to find opponents and also the player welcoming the new player is awarded with points (if he wins) in case the new player just leaves.
It means especially tier 1 and tier E will have an easier time maintaining their ranking, but does that really matter? There will also be more tier 1 and tier E players which in turn will benefit tier M players as well