I've used each of the above, at different times. (Actually, I use expr and b(); rather than &&.) Which one I prefer at any given time depends on the actual expr and b(), the surrounding code, and the intended audience.

expr and b(); is good when the expr is more significant than the call to b(), or when you're emulating a switch statement.

b() if expr; is good when the call to b() is more significant than the expr, such as b() if $DEBUG;.

if ( expr ) { b(); } is good because it has the clearest structure, especially for people familiar with other languages.