に加えて・のみならず・おまけに: the difference

Meaning

Not only...but also, In addition, Not to mention

Past papers

Not yet

Example

今学期こんがっきから文法ぶんぽうに加くわえて漢字かんじも教おしえることになった。

Starting this semester, in addition to grammar, I will also be teaching Kanji.

Meaning

Not only...but, Not just...but, What is more

Past papers

Tested 2 times in past papers

Example

炭素たんそ繊維せんいは極きわめて丈夫じょうぶであるのみならず非常ひじょうに軽かるいらしい。

Apparently, carbon fiber is not only extremely durable, but also very light.

Meaning

In addition, On top of that, What is more, To make matters worse

Past papers

Not yet

Example

家いえを掃除そうじした。御負おまけにランチを作つくっといた。

I cleaned the house. On top of that, I also made lunch.

How to tell them apart

「に加えて」: adds a new fact onto an existing one; common in writing and news.

「のみならず」: 'not only A but B too'; the tone of an essay.

「おまけに」: casual; starts a sentence to add one more thing.