Just to help, here’s some background details:
My team is a 3rd grade boys travel team.
In our league, for 3rd graders, they are only allowed to play ½ court man to man defense. No pressing ever.
We play 18 minute halves.
I have 8 players on my team.
I don’t really have an assistant so it’s just me on the bench.

In the past I’ve coached rec ball so it’s a little different, but something I struggle with is defensive assignments when players go into games. This may sound simple, but for me it hasn’t been. If I just tell them to pick a guy, I end up with 3 on the same guy and bad matchups. if I tell them 3s of positions, they seem to revert back to almost a zone because they associate the # was a zone from when they were taught that early on. Sometimes hard to have players coming out say who they had if the other team subs as well. It’s ok if we have possession because having them guard who guards them seems to work. Any tips, ideas on making this an easier or smoother transition when subbing and even when starting the game? Thanks in advance for your help. Hopefully my question makes sense.

Coach -

A couple things that will help...

Read this post for some suggestions that helped us:
http://jeffhaefner.com/coach/youth-defense-how-to-make-sure-young-players-know-who-they-are-guarding/

Practice more man defense and shell drill in practice. It sounds to me like the players don't have a good understand of man defense yet. You can dig around for free resources on how to build a really good man to man or you can get something like this if you really want to do things as effective as possible.
https://www.breakthroughbasketball.com/pr/huber-defense.html

Also, in practice require your players to call out who they are guarding every time the run down the court in transition defense, every time they go in, and dead ball situations. That's a really good habit to get into.

Hope this helps.