Zero.
Object reference diagram looks like this:
You can see that the reference is circular. A reference from main to m3 keeps the m3 object alive. In turn, m3 keeps alive m1, which keeps m2 from GC.
Note that if you set m3 to null, all three objects would become eligible for GC at once, despite circular references existing for each one of them. GC is smart enough to figure out that all references are coming from GC-eligible objects, and collect all three.