Употребите Past Perfect после союза, сделав соответствующие изменения в главном
предложении.
1.If they invite me, I shall go with them.
2.If I feel better, I shall go with you.
3.If she saves her money, she will be able to go on a vacation.

В третьем вместо should - would нужно написать

If they had invited me, I should have gone with them.If I had felt better I should have gone with them.If she had saved her monly she should have been able to go on a vacation.