In your code there are to many unnesseary () that corrupt your expression.
The following works on my system.
Expr1:
IIf(IsNull([Actual_RL_3rd_Reimport]),
IIf(IsNull([Actual_RL_2nd_Reimport]),
[Actual_RL_1st_Reimport],
[Actual_RL_2nd_Reimport]),
[Actual_RL_3rd_Reimport])
Identing your...