I =(us 1+US2-US3)/(r 1+R2+R3)
The second question:
(r 1+R3)im 1-r3im 2 = us 1
-R3Im 1+(R2+R3)Im2=US2
For the column grid equation, the rule is:
Self-resistance * current in this battery+mutual resistance * current in adjacent batteries = algebraic sum of voltage rise of voltage sources contained in this battery along the current direction.
Wherein the mutual resistance is the negative value of the common branch resistance of the two meshes.