Why did montesquieu want the government structure to have 3 branches

To separate each branch from being over powered and each law to be passed down from branch in order to become valid, for each branch to be responsible for its own aspect. (ex) Judicial = supreme court , jury Etc, 

So that each branch was unable to have more power over the others.

If u can translate that to english i will surely you!

Assemblies do not have written codes of laws. laws are matters for magistrates and courts, not assemblies. codes of law are things such as civil law, criminal law, and constitutional law. what assemblies have are procedures regarding debating, voting and other business they might carry out.  also note that rome had three assemblies: the assembly of the soldiers, the assembly of the tribes, and the plebeian councils.