Grammatical mistakes in sentences

Hi there!

I need help at a task on grammar:
Find and correct the mistakes, if any, in the following sentences:

1.The meeting had been supposed to last no longer than one and a half hours but turned out to be a four houraffair involving plenty of excitement and even free sandwiches.

2.Come on, let me show you the rooms we have been painting today.

3. Mark my words, this president will have been destroying his country’s democracy before the year’s out.

Thanks for helping!