There are two factors at play here. Line of sight and Clear path to target (what previous editions called Line of Effect).
Line of sight is only required for the first target. It's not specified as required for the secondary targets so you should not require it.
From "Targeting" on page 204 of the PHB:
To target something, you must have a clear path to it, so it can't be behind total cover.
If you place an area of effect at a point you can't see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction.
So a valid target on the first part of spell is something you have a clear line of sight to, and also a clear path towards. The secondary targets do not need a clear path from you, but from the initial target since that's the point from which they emanate.
As far as your reading of the spell with respect to secondary targets, you're reading "As many as" incorrectly. This means "up to 3" not "exactly 3", so if you only have 2 available targets (that you want to hit), you're welcome to do so. While three bolts are created, they do not all need to be targeted and can be assumed to ground harmlessly.
The targets are chosen by the caster as it's not specified they are chosen randomly.
That depends on if it is casting a spell, or granting a spell-like-effect.
To cast an actual spell from an item, it requires your concentration. This is specified on page 141 of the DMG
[...] The spell uses its normal casting time, range, and duration, and the user of the item must concentrate if the spell requires concentration.
This rule would be used in the case of an item such as a Staff of Fire, which allows you to use it to cast the spell Wall of Fire (which requires concentration).
However, to cite the specific example you gave of a Ring of Invisibility. No.
Again, per the DMG, page 141...
Many items, such as potions, bypass the casting of a spell and confer the spell's effects.
The rule for concentration applies if the magic item allows you to cast a spell. It does not apply if the magic item grants you a spell-like effect. The stats on a Ring of Invisibility say
While wearing this ring, you can turn invisible as an action.
It does not say you can cast the spell Invisibility...just that you may turn Invisible as an action. This is an important distinction.
Thus, a Ring of Invisibility does NOT consume your concentration as it grants a spell-like effect, rather than allowing you to cast a spell. A Ring of Water Walking is another example of this, in that it allows you to walk on water at-will, rather than allowing you to cast the spell Water Walking.
Best Answer
No
The rules state:
These bolts must (presumably) be fired from the previously mentioned +1 hand crossbow and each "casts X on a target hit with the bolt" and explicitly have "a duration of 1 minute."
Who's casting the spell?
These can be distinguished from items that "allow the user to cast a spell from the item" and force "the user of the item must concentrate if the spell requires concentration." All items that allow you to cast a spell explicitly say "you cast" - these items don't say that, they say the item casts.
The items themselves "casts X on a target hit with the bolt"; they do not "allow the user to cast a spell from the item."
Are these Concentration spells anyway?
The rules say in respect to spell casting items:
These items change the duration from, for Hold Person, "Concentration, up to 1 minute" to "a duration of 1 minute."
These items, therefore, do not require anyone or anything to concentrate.