Set up a F to S relationship between both task A and B and Task A and C.

Set Must Start on Constraints to B and C. Change the constraint on Task A to As Late As Possible to calculate when A must start.

The issue with this scenario is you need to know with certainty that Task A only takes 20 days.

Personally I would leave Task A with an ASAP constraint to build in some buffer against inaccurate estimates or employee illness or vacation.