I've just been warned by mercs that if I don't make amends in 3 days, I'll become enemy of NCR. Don't want that!

How can I counteract this and change my reputation from shunned to, at least, neutral?

Thanks!!

You can wear any ncr armor and it will set your reputation back to neutral as long as you wear it. Also if you haven't you can get a pardon from the ncr right when you kill benny and talk to ambassador crocker but you don't have to do the first mission just accept it and your set. Anyway once your reputation goes down you can't bring it back to neutral or anyhting but if you do side quests for the ncr. Your reputation will go up to where they won't want to kill you but you can never get the reputation of liked or idolized.

Well, find ambassador Crocker at the strip, and work for him. That normally helps gain good karma with the NCR. Should get you to neutral, but it might make you vilified with the Legion if you continue far enough, making them send legionary assassins, you will not give you three days to be better to the legion.

If you have not made it to the strip yet, go there as fast as possible. After you enter and do one or two quest, your karma with NCR and Legion is basically set to neutral (unless you are positive with them already).

WOW!! That's Fast!! Thanks so much... I plan on setting things straight with the military faction and you gave me good hints to succeed.
I'll forget about positive rep though. Maybe in a new game, I'll be careful not to help Powder Gangers at the Prison...

Haven't found the strip yet. But will look closely to it. Wearing the NCR uniform for the moment while near them.

Just started this week. Yes, it's bugged. But so far, I'm having fun and it really brings you back to the old games (which are some of my all-time favorites).
And conversations are closer to originals than F3.
Having spent - gulp! - more than 500 hours on F3, I expect about the same on NV.
Bethesda will kill me with these wee hours of night playing...