i hope your money is ready. :V
Anyways, Jr. has several advantages going for him over Luigi (I'm not as familiar with Olimar)
- Outranges him with disjointed aerials.
- Good stage control with mechakoopas (down B)
- Excellent strings off of his kart dash (side B). Also a huge boost to Jr's mobility.
- Stronger kill moves in his smashes, and Up B.
- Better edgeguarding game.
- Arguably better recovery unless you can mash cyclone without your second jump.
- Heavier, will survive longer.
That being said, Luigi can seize on several of Jr's weaknesses...
- Jr's heaviness doesn't help him escape Luigi's crazy grab combos. If you read the DI correctly you can get massive damage in on Jr.
- Cyclone barrels through many of Jr's attacks, but don't abuse it because it's still punishable.
- Jr's grab is super slow, so shielding many of Jr's options is safe and lets you punish. Watch out for shield pressure tactics cuz if Jr's offense isn't stopped, he will break your shield and kill you.
- Your Nair can interrupt Jr if he has poor spacing.
- Fireballs can stop Jr's approach if you hit Jr himself (the car has super armor on his side B, so hitting it from ground level won't help. If you short hop and fireball you should be able to hit Jr.)
- You can pick up the mechakoopas and throw them back at Jr.
Since you don't have much MU experience, you should try to find a few Bowser Jr. players to fight (check the player finder thread) and do your best to get some decent practice in before your money match.
Good luck!