Don't get Destiny on the ps3.

To answer your questions: 1) Solo or with friends? You can only get so far playing this game solo. Playing solo you can enjoy the story missions, but the story is not complete, the story will not end anytime soon. Technically you can solo most things, but you need good gear and the best way to get good gear is to play consistently with groups. There are activities that are best played in groups, but there are methods to solo some of the game's end game activities (although the average joe fail a lot).

2) How active is the PS3 version? Truth be told I have no idea. You can try looking at /r/fireteams for PS3 players (very unlikely), you can google search for a destiny looking for group aka LFG website, or you can try to look at the100.io to see if there are active players.

4) Is the Destiny The Taken King Legendary Edition the definitive edition? No. Newer expansions are being released. The Rise of Iron Expansion was released for PS4 and X1. The Rise of Iron and future expansions will not be released for the PS3 and 360. Refer to this: https://www.bungie.net/en/Help/Troubleshoot?oid=44808

4) Should I play it? (You technically didn't ask this.) Its a subjective question. It depends. I wouldn't recommend playing solo. The gunplay is amazing with the magic headshot lock.