Splitting AMS alignat environment into ams alignat and ams multiline in lyx

I have three lines of equations in ams alignat (for example):
line 1: a = b
line 2: a^2 = b^2
line 3: a^3 = b^3
(I apologize for the bad formatting but mathjax doesn’t seem to work here)

How do I separate lines 2 – 3 from line 1 so that I can keep line 1 as alignat and lines 2-3 as multiline?