How to use expr inside if statement?

I have not managed to understand how to embed expr in other constructs. I can easily type

set i {1 2 3}
expr {[llength $i]} #result is 3

However after long research I have not managed to find a way to put that inside if

if {"magic tcl code with expr and llength==3 required here"} {
   puts "length is 3"
}