ella juega ( marked wrong because it should be juegan... but isn't 'she' third person singular. and not they?

Hola Elizabeth

I think you may have misread. There is a question saying "Ellas ....." but not "Ella... " in singular:

Ellas ___ con la arena en la playa. (They play with the sand at the beach.)

It is the plural "they feminine".

Saludos

Inma