Using your code:
switch(true)
{
case (something):
{
break;
}
case (something2):
{
if(check) {
something3 = true;
}
else {
break;
}
}
case (something3):
{
break;
}
}
This will get to case something2 and run your check. If your check passes then it doesn't run the break statement which allows the switch to "fall through" and also do something3.