I'm not sure what output you're seeking, so I'll just present a bunch of
options:
NB. Prefix a row of A to each B
a ,&.> b
+-----+--------+--------+
|1 1 1| 2 2 2| 3 3 3|
|0 1 2| 4 5 6| 7 8 9|
|3 4 5| 7 8 9|10 11 12|
|6 7 8|10 11 12|13 14 15|
+-----+--------+--------+
NB. Prefix a col of A to each B
a ,.&.> b
+-------+----------+----------+
|1 0 1 2|2 4 5 6|3 7 8 9|
|1 3 4 5|2 7 8 9|3 10 11 12|
|1 6 7 8|2 10 11 12|3 13 14 15|
+-------+----------+----------+
NB. Replace the top row of each B with an A
a (, }.)&.> b
+-----+--------+--------+
|1 1 1| 2 2 2| 3 3 3|
|3 4 5| 7 8 9|10 11 12|
|6 7 8|10 11 12|13 14 15|
+-----+--------+--------+
NB. Prefix an A to the first row of each B
a (}.@:] ,~ [ , {.@:])&.> b
+-------+----------+----------+
|1 0 1 2| 2 4 5 6| 3 7 8 9|
|3 4 5 0| 7 8 9 0|10 11 12 0|
|6 7 8 0|10 11 12 0|13 14 15 0|
+-------+----------+----------+
NB. Substitute A for the first item of each B
a (<0;0)}&.> b
+-----+--------+--------+
|1 1 2| 2 5 6| 3 8 9|
|3 4 5| 7 8 9|10 11 12|
|6 7 8|10 11 12|13 14 15|
+-----+--------+--------+
NB. Same using {. }. [ ]
a (}.@:] ,~ [ , }.@:{.@:])&.> b
+-----+--------+--------+
|1 1 2| 2 5 6| 3 8 9|
|3 4 5| 7 8 9|10 11 12|
|6 7 8|10 11 12|13 14 15|
+-----+--------+--------+
-Dan
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