1) Since you don't give the error message, it is hard to say what the problem is. In general, I would recommend avoiding very complex command lines like this, and do it in 3 steps: 3dcalc as needed, 3dttest++, then delete the temporary files

2) In general, it is not true that regressing with the average of 2 regressors gives the result that is the average of the separate regressions. This happens because the result of the regression is not a linear function of the regressors -- it is a linear function of the DATA. So if you have 2 separate (perhaps similar) conditions that lead to Brick(1) and Brick(3), and then want to see where ignoring their differences leads to changes from Brick(5), in my opinion you should average Brick(1) and Brick(3) and then test vs. Brick(5) -- as you seem to be trying to do. Otherwise, you are carrying out a separate analysis with the merged #1 and #3 regressors, and then how do you compare that to the analysis results from the separated #1 and #3 cases?

In the case that the regressors from #1 and #3 are orthogonal and have the same L2 norm, then the results of the 2 procedures would be the same. This would be the case if the 2 regressors had the same number of stimuli, and the stimuli were separated in time enough so there was no overlap between the responses.