The correct F-contrast fro the main effect of time would be:
[1/2 -1/2 0 1/2 -1/2 0 zeros(1,15); 0 1/2 -1/2 0 1/2 -1/2 zeros(1,15)]
This will test if any of the time points are different from each other.
The "main effect" of time in group A1:
[1 -1 0 0 0 0 zeros(1,15); 0 1 -1 0 0 0 zeros(1,15)]
This will test if any of the time points are different from each other in group A1.
If you want a decrease over time in group A1, then use a T-test to get direction:
[1 0 -1 0 0 0 zeros(1,15)]
This contrast only requires that B3<B1, which would make a negative slope irrespective of B2. If you want a decrease from B1 to B2 and B2 to B3, then you need multiple contrasts and to combine them after thresholding.
Hope this helps.