How do you use 如此 (ruci)? Is it the same as 那么 (name) or 这么 (zheme)?

如此is more often seen in literature, formal situations, written Chinese, etc.
If someone say (by say, I mean spoken Chinese)你怎么如此笨, it sounds a little geeky.

In our daily life, 这么and那么are more commonly used.
你怎么这么聪明？ 这to you
他怎么那么聪明？ 那to others

Recording to my experience ,we do not have any problem using them in the same way.for sure,sometimes you can use them as adv，all here mean "so"。
如此的无知/那么的无知/这么的无知=so ignorant
lily is good student that she always does whatever her chinese teacher said.she is 如此(这么，那么）absorbed in learning chinese ,which makes her the best one in the chiese course.

"如此" is less 口语化 than "这么" and "那么"
so although 你怎么这么笨 = 你怎么如此笨 is true, we seldom say "如此" in spoken Chinese.