Blue Sentinel with Svellsongur, does it give an additional trigger of hold effects?

Yes — with Svellsongur attached, Blue Sentinel gives two additional triggers of each hold effect (total 3), and you'll have two separate "When I hold" abilities, each triggering 3 times.

Here's how it works:

Svellsongur copies Blue Sentinel's printed text to its own Effect Text. Per Rule 718.3 (and 136.2.c), that Effect Text is appended to Blue Sentinel's Rules Text while attached. So Blue Sentinel effectively has two instances of both its passive and its "When I hold" ability.

Two passives stack additively. Each instance of "Your hold effects for holding here trigger an additional time" is a separate modification. So each hold effect triggers 1 (base) + 1 (first passive) + 1 (second passive) = 3 times instead of the usual 2 with just one Blue Sentinel.

Two separate "When I hold" abilities. The original printed one and the copy from Svellsongur are two distinct triggered abilities. Each is a hold effect, so each triggers 3 times due to the dual passive. That's 6 total [Add] [A] triggers — you'll add 6 Power at the start of your next Main Phase.

Same applies to any other hold effect at that battlefield (e.g., Grove of the God-Willow's "draw 1" triggers 3 times, drawing 3 cards).