What do the triple circles in a triangle configuration on the left side of each player tag signify? They come and go during the game, but I haven't figured out what they mean. Also, what does the 3 under mine mean?
What Wondercricket said: The top-most three-circle-icon next to your shield and health numbers is shown if you are in affinity sharing range with another squad member. The number 3 below that tells you how many squad members are in range to share affinity. If one of them is out of range then there is no icon next to their name.
The wiki page for Affinity says:
Be near an allied Tenno when they kill an enemy. When within 50 meters of an ally when they kill an enemy, you gain the same total Affinity, split according to your own weaponry. This doesn't reduce the Affinity gained by the killing player, and doesn't reduce the Affinity gained by other nearby allies.