1: low health player under tower, you dive, you end up dying, but your laner gets the kill is this worth?
2: your at low health, have no flash, along with 2 full health teammates, and being chased by three other players. should you just fight them, even if you are guaranteed to die?
1: As far as upfront statistics go it is slightly worth due to your team getting a kill and assist while they only get a kill.
The problem is it is using up the time of both you and your laner to only get the advantage of an assist and possibly some free farm for your laner depending on how low the dive took him. If the enemy laner was low health he was in the position of either going back and giving your laner free farm and damage on turret anyway or staying and trying to farm under turret. This already puts the enemy laner at a disadvantage and so killing yourself just to finish them off isn't the best option. The enemy jungler is also free during this time and most likely farming while you only get an assist for the dive. This also opens you up to getting counter jungled. Basically in the scope of things this scenario isn't going to be worth for you if the enemy jungler capitalizes on it.
If the enemy laner decides to stay in that position he is most likely waiting for a gank which could set up a great counter gank opportunity for you. If he recalls you could leave it at that, your laner is getting an advantage by forcing the enemy to recall, or you could also immediately help your laner attack the turret if you think the two of you can take it before the enemy laner gets back.
2: I feel this one is very situational. I would say more often than not it isn't going to be worth to fight them knowing you are going to die. If you know they are gonna catch up to the three of you and there is no chance of escape it's worth to fight if you are able to stall long enough for your teammates to get away. Usually though using up as much of the enemies time as possible is a good idea, especially if there is a chance teammates can make it to you and turn it around.
Your best bet is to play it safe and survive,but trading support for an adc is ok I guess.
It's okay to die in tower dives, however, if you get shutdown from the towerdive, it isn't worth.
If your low on health, don't have an escape, and the enemy can't catch you, you may as well run. However, if the enemy has someone like Quinn or Nidalee, champions who have a way to chase you down, you can sacrifice yourself. If they are going to eventually catch you, its better you stop them and die, and they don't get all three of you.
TWO (cus writing 2 is somehow turned into 1, cant turn off auto listing): at first I thought you meant only you would stay to fight, in order to help them escape, then yea, that's a thing, if you know for sure you can't win the 3v3 if you mean you as in all of you, then it really depends, and in the end if you're the only 1 dying but you know they have the skill to get 3 kills off of your help, totally worth
if the enemy laner dies, you die, and your laner gets the kill it's worth, because they then lose out on an additional minion wave of gold and exp. Furthermore, you more than likely get the assist gold.
If you know you can win the fight, there's a lot of variables and game knowledge but yes if you can turn it.
