i’m aware people say morning instead of good morning, do you also just say evening or afternoon?

Colloquially, this is very common. Afternoon, ma’am. How’s your day been so far? Evenin’, hoss. Cattle give you any grief today? I knew a guy who used to just say “Day” as a greeting because I said that to him once as a gag and he thought it was funny and kept doing it. But this one *isn’t* common. Don’t go around saying “Day” to people. Or, conversely, please do. Lmao.

Yeah I have heard people said it. It's the same case as 'night vs good night.

I think this is the type of thing that varies a lot by region/class/race/etc. I would say "mornin'" but would basically never say "good evening" or "good afternoon" (including just afternoon or evening) except in very formal circumstances. Depending on who I was talking to, I would say "hey how are ya" "sup ma' (man but I would elide the n)" "hello" "how's it goin'" or just smile.

U could yeah

In the US this is fairly common for any context where you would say "good morning", possible exceptions for a spouse/partner who you might greet early in the morning but may just continue that into the rest of the day even if work / job / errands /etc happen in between. But for guests, students, answering the phone, customers, riding a bus or taxi, etc. you would use "Good" plus the relevant time of day. "Good night" is usually a parting phrase rather than a greeting, but is also very common. If you are working overnight you would greet your coworkers with "hello" or "good evening!".

Haha in Australia we even shorten “Afternoon.” To just a quick “Arvo” and it is absolutely fine for passing by neighbours or strangers. Even preferable as casual and friendly

Absolutely. In my part of England it’s very common!

Some do but I personally only ever say "morning". "Afternoon" especially sounds a bit odd to me.

yeah. It's similar to how you can shorten "buenas tardes" to just "buenas," but in english you drop the opposite part of the phrase. It becomes more casual but the meaning is the same.

Yes. For me, depending on how good the day/afternoon/etc. was, I’ll say good. If not, I’ll leave it out.

None of them are uncommon. But where I’m from, “[good] morning” is much more common than “[good] afternoon” or “good evening.” That’s because here, “[good] evening” and “[good] night” feel much more formal and old-timey than “[good] morning.” I never say “good afternoon” or “good evening,” ever, personally. I write them in emails all the time.

Yes.

Afternoon, yes. Evening, err idk

yes

Sometimes we even reply, Law don’t go round here, law dog. Understan?

Sometimes. 😀

Since others gave reasons, I'll simply add onto it. Modern day English has this laziness, or convenience, to it in some cases. Other languages do it, too. Spanish, for example, is like this but the other way around. Instead of, let's say, Buenos dias (Good morning), some informally say "buenas," meaning simply 'good.' To add onto u/parsonsrazersupport about region, class, etc. I rarely hear evenin' or afternoon. I do hear just those, but a lot of people in my region tend to say other, informal things. Heck, even I will give a head nod with a "Sup?" and sometimes without a single word.

Yes.

It's more common to say "Mornin" without the ending G, at least here in the US.

Yep, I’ve heard Morning and Afternoon more. I think Evening is a more southern thing but I may be off base here

Yes

Yes. You can also say "night!" on your way out the door.

The short answer is yes, we can and do say all three, your meaning and intentions would be fully understood, and it would be socially acceptable and adequately polite (although if you were meeting dignitaries or you were at a formal ceremony you should say the whole phrase). Longer version: In the US, we say “morning” much more frequently than “afternoon” or “evening.” Saying “morning” would be extremely standard and neutral, whereas saying “afternoon” or “evening” as opposed to “g’d’fternoon” or “good evening” would often (but not always) imply that you’re stressed, rushed, or being brisk, unless you put on an extra bright cheery voice to say “afternoon!” or an almost sing-song-y devision of syllables to say “eve-ning” with a smile. BUT, despite what I said in my longer explanation, it’s still totally acceptable to say in any way. I was just providing additional context. (US, Millennial, female, graduate degrees, living in the US Mountain West but with significant childhood cultural exposure along the East Coast)

Yes

Morning! Nice day for fishing ain't it! It's just a habit, but yes, you can say "Evening!" too, while you tip your hat 🎩

Yes, just as instead of 'good dog' you will hear simply 'dog'.

If someone says “morning” to you, it’s perfectly acceptable to say “good morning” back to them. It’s just the way they say it, not intended to be offensive or anything because it lacks the word good in front of it.

I just want to note, it's not that I'm just saying "Morning!", it's just that I'm reducing the unstressed "Good" to either a quiet "guh" sound or simply an unvoiced exhale. But it's there in thought every single time. 

Some people are in fact not having a good morning or good afternoon. Potentially even having a pretty bad day. "Morning" and "Afternoon" take out any implied positivity that may not actually apply.