Just for reference so someone else doesn't spend 2 hours on this...
Using d3 linux 7.2 or d3 nt 7.4

Tclread and user exit u0003 are supposed to be interchangeable but
u0003 has a bug and tclread does not.

If you assign a value to a dynamic array with more than 1 attribute
mark, prior to executing the u0003 user exit, the value returned by the
user exit will always be the last value in that array instead of the
command line info. Tclread does not have that issue.

Observe:

001 G = 'VALUE IN G<1>'
002 G<2> = 'VALUE IN G<2>'
003 ARGS = OCONV('','U0003')
004 PRINT ARGS
005 TCLREAD ARGS2
006 PRINT ARGS2
eoi 006
..FI
[221] 'TEST' filed.
:BASIC N TEST
TEST
..
[241] Successful compile! 1 frame(s) used.
:RUN N TEST
VALUE IN G<2>
RUN N TEST
:

Granted, reading the tcl arguments should probably be done first, but
it's not always practical when using include code, etc.