そういえば・ところで・それにしても: the difference
そういえばN2
Meaning
Speaking of which, By the way, Come to think of it
Past papers
Tested once in past papers
Example
そういえば、今日は母の日だ。
Come to think of it, it is Mother's Day today.
ところでN3
Meaning
By the way, Incidentally
Past papers
Example
ところで、どこに行くつもりですか。
By the way, where do you plan on going?
それにしてもN2
Meaning
Nevertheless, Even so
Past papers
Example
ラッピングをお願いしたが、それにしても、かなりの過剰包装だ。
I asked for wrapping, but even so, the wrapping was excessive.
How to tell them apart
「そういえば」: the new topic must be linked in your mind to what was just said; the remark woke up a memory.
「ところで」: a topic changer, 'by the way'; it can have no link at all and jump to a fresh subject. 「今日の天気はいいね。ところで、来週の件だけど…」
「それにしても」: mostly a concession (even so, that's too much), with surprise; when it moves on, it is to your own impression (「それにしても、大きくなったね」), not to something you just remembered.