I have used all three of them extensively, but now I only skateboard. I used to do a lot of rollerblading but it wasn't very useful outside as a means of getting around, so I mainly did it at rollerblade arenas. I loved my scooter until I lost control of it going down a hill and I fell off and it went into oncoming traffic and obviously broke. My friend advised me to get a skateboard as they can be cheaper, and are a lot more portable than scooters. If you have a skateboard you can take it indoors, which is very nice. As for detail about why rollerblades aren't good outside, it is more difficult to stop than on a scooter or skateboard, and you have to actually remove them if you want to stop rolling. When I go skating there are times when I don't want to be on a board, like when I'm in a shop. So unless you brought a pair of regular shoes with you while rollerblading, going indoors is out of the question. Also, having to take them off every time you want to just walk is a pain.
Honestly it depends. If you want to go the easier route, definitely go for the scooter. Roller blading is also fairly easy, but finding a place to do it is not. Most of the time you would have to go to a park or a roller rink. Skateboarding is a little bit difficult, but after a wee bit of practice, it is really fun. Also, skateboarding makes you look cooler.
Inline skates (rollerblades). I have all three but I'd choose inline skates. I don't like my scooter, it doesn't run smooth and I'm not really good at skateboarding.
Scooter always.
