Can I change my bike from a 3x7 to a 1x7?
Changing to 1x7 is not going to give a very good experience.  There are several features that work together to make a nice 1x setup and none of them are present in 7 speed groupsets.

1x first became mainstream with the introduction of wide range 11 speed cassettes.  This allowed an adequate range of gears without the jumps between becoming uncomfortably large.

However 1x also presented problems with the chain being dropped over rough terrain.  The clutch mech and narrow-wide chainring were developed to combat this.

So whilst it may be possible to convert to 1x7, it is certainly not advisable.