Trouble with a5- e1 diaganol

In a couple of recent games where i had trouble with an attack along this diaganol, Both games i was playing white, one i played the queens gambit the other i played against the french defense. Understanding that these are probably weekness to the openings i played, could i still get some pointers on how to defend these weekness

For example a possition like this

play a3 before he can bring his bishop there

Even with a3 Qa5 then Bb4 (you cant take because then free rook) 

Try Nc3 followed by o-o.  Later develop QB to Q2, K3 or B4 depending on what Balck does

There is no problem in this case. You can play Nc3 or Bd2, and nothing bad happens. In some positions a diagonal can be week, really, but it is not that position.

I don't see what's wrong, honestly. Nc3 or Bd2 covers things up, and then you castle. So what's the problem? There's nothing to worry about. 

If you're talking about something like the Cambridge Springs defense to the Queens Gambit then I can understand troubles with that diagonal. But here it's not, and you have it covered down.

Nc3 is fine indeed. Your primary concern as white should not be a pointless check from b4, but rather the other bishop sortie, ...Bb5, which aims at exchanging Black's "bad" bishop with your "good" one.

That's also a thing to be aware of taking account of the unusual Bd7 move by black. However, it can't be stopped if your opponent does want that. 

idk maybe i was just making blunders i will review the games a little more closely

actually Bd2 loses a pawn if ur not careful (of to take the exchange with the queen) which is wat i think happened in the game. thanks for the help

As others have said, Nc3 is fine in the diagrammed position, blocking the check without losing material. If black insists on trying Qa5 at that point to pile more pressure on the knight, then Bd2 can be played. Maybe post the whole game, so you can get some more analysis of the entire game.