Add progress update for Checkpoint 3

PROGRESS.md

@@ -1,5 +1,33 @@
 # SE 3250 Progress Checkpoints
 
+## Checkpoint 3
+*28 Nov 2025*
+
+[github.com/SnowSE/final-project-KylerOlsen](https://github.com/SnowSE/final-project-KylerOlsen)
+
+[Compare SE3250-Checkpoint2..SE3250-Checkpoint3](https://github.com/SnowSE/final-project-KylerOlsen/compare/SE3250-Checkpoint2...SE3250-Checkpoint3)
+
+**Report**:
+
+Since checkpoint 2, I completed `token string` parsing and started working on
+the REPL. The lexer and REPL are basically finished as much as I am going to do
+for this assignment. I did start to implement builtin functions and operators.
+Currently only
+[`depth`](https://sls.purplecello.org/complete_operator_reference.html#depth),
+[`drop`](https://sls.purplecello.org/complete_operator_reference.html#drop), and
+[`/`](https://sls.purplecello.org/complete_operator_reference.html#_5)
+
+**Plan**:
+
+By the final turn-in on December 6th, I plan on getting all arithmetic
+operations working. And the python and rust ports with help from AI.
+
+**Stuck**:
+
+I'm not really stuck on anything, but I am running out of time, plus I am
+worried about not having enough AI usage to do the ports. If I at least get the
+main structures of the ports, I hope it will be enough for me to finish.
+
 ## Checkpoint 2
 *20 Nov 2025*