Recently I found out that my single coil at the neck has much louder sound than the humbucker at the bridge. The middle single coil is the same as the neck one. Is this normal? Do I need to make the humbucker higher?
Here's a picture:
It's not normal in the sense of being (usually) 'what you want', but it is something that you have to deal with in multi-pickup configurations.
It's not necessarily that the neck pickup is more powerful in itself; it's that the strings vibrate further from side to side at the neck, as they're nearer the middle of the string, so they induce a more powerful signal in the neck pickup (the ends of the string are anchored and don't move, so the points near the ends don't move very far). The difference due to pickup location may be much more than the output difference between a given humbucker and a given single coil.
It's common to use a much higher output pickup for the bridge location - and/or raise it higher than the neck pickup (to compensate for this phenomena) and get a more balanced output between all pickups.
Most likely the difference in volume comes from a difference in output--some pickups are made to be "hotter" than others, and they'll be adjusted differently according what position on the guitar they're intended for.
Another factor that could affect output is if the humbucker has a coil-splitting feature, which allows you to achieve single-coil sounds, but at reduced output. Hope this helps!
A humbucker has two coils connected in opposing direction. That's how it bucks the hum, bu having the respective effects on the coils cancel. The magnets are also oriented in opposing directions so that there is a net pickup effect, but there are still components partly working against one another.
As a result, humbuckers tend to be quieter than their single coil counterparts, but the net result, even when you turn up the volume accordingly, tends to have less hum and tendency for magnetic feedback.