Add 3MT-3digit: exhaustive 3-digit addition (500,500 examples, 3,003,000 tokens)

3MT-3digit/test-00000-of-00001.parquet

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:1bdf324c4635f701fd3b5333599b6694f81f4aa9bfe214aa7b6fff12a6124fbc
+size 617479

3MT-3digit/train-00000-of-00001.parquet

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:dbd2b41bb50805d06fe7e22d213c97cff5c11f944a98853c31a23ed032f18b6b
+size 4790243