The first is okay.
The second has two errors of capitalization and used the wrong form of the verb. Some may also say it has two errors of punctuation.
The third uses the wrong tense.
Hi,
please assist, Are the below sentenses are corrected?
He or she is a thief.
My Pupils as well as i am doing this work.
One of the cats scratch the children.
Regards,
The first is okay.
The second has two errors of capitalization and used the wrong form of the verb. Some may also say it has two errors of punctuation.
The third uses the wrong tense.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.