You couldn't select the result of an expression else only dimensions. But you could create a calculated dimension from it - for example the following within a listbox by choosing expression instead of a field:
aggr(if(len(Table1)+len(Table2)+len(Table3)=9,'Yes','No'), [Project Code])
Otherwise you need to create this as a field within the script, maybe with something like this:
load distinct [Project Code], 1 as A from source1;
load distinct [Project Code], 1 as B from source2;
load distinct [Project Code], 1 as C from source3;
load [Project Code], if(rangesum(A,B,C) = 3, 'Yes', 'No') as Synchronized resident t1;
Depending on your requirements and your data might a similar approach with mapping more suitable: Mapping as an Alternative to Joining.