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B and C pull A to the right. Use the universal law of gravity
F = G*M1*M2/R^2
to compute the individual forces and add them. You must have seen that equation before.
For B, A pulls it to the left and C pulls to the right. Make sure the force due to A is subtracted from the force due to C.
You should be able to calculate the force on C after doing the other two. Both forces (due to A and B) will be to the left.