if or elif either true then do something

You could also omit the either_true flag completely if doSomething3 is a single line of code (e.g. a function call):

if x:
  ..do something 1
  ..do something 3
elif y:
  ..do something 2
  ..do something 3

It maintains the nice property of evaluating x and y at most once (and y won't be evaluated if x is true).

I would handle this by using nested if statements i.e.

if x or y:
    if x:
        ...do something1
    elif y:
        ...do something2
    ...do something3

As some comments have pointed out, the best solution will depend on what x & y are. If easy readability / concise code is your goal then this or other answers given should be fine. If however x & y were expensive function calls then it would be better to do something more like what you have done to avoid calling the function twice.

You could wrap some of it in a function:

def do_stuff():
    if x:
        ...do something1
        return True
    elif y:
        ...do something2
        return True
    else:
        return False

if do_stuff():
    ..do something3

Or all of it in a function:

def do_stuff()
    if x:
        ...do something1
    elif y:
        ...do something2
    else:
        return

    ..do something3

if x or y:
    dosomethig1() if x else dosomething2()
    dosomething3()

Of course, this does evaluate x.__nonzero__ twice. Usually that's not a big deal, but if it is expensive, you can always evaluate that upfront and save it to a temporary variable.