Unknown protection?

Hello,

I would like to know how you should attack a packed file that PEid don't find the protection.

When I want to open it with Ollydbg the program don't stop at the EP but break on a INT3 exception. After it's a sequence of exception and finally the program stop.

I think all the protection it's around anti debug but I'm not sure, and I don't know how the program can run without break at OP.

Please give me some tricks to begin