Does Block Shield 3 work with Svellsongur to give an additional Shield 3 to the unit defending?

No, Svellsongur does not copy the Shield granted by a card like Block.

According to the official rulings and FAQs for Svellsongur:

• Svellsongur only copies the printed text on a unit card. It does not copy text, keywords, or abilities granted to a unit by other sources, such as spells (like Block), other equipment, or battlefield effects.
• Because Block is a spell that grants Shield to a unit, that Shield is not part of the unit's "printed text." Therefore, Svellsongur will not copy it.

Regarding Shield Stacking: While Svellsongur cannot copy the Shield granted by Block, it is important to note that if a unit has its own printed Shield keyword and is also targeted by a Block spell, those Shield values do stack additively (as per Rule 814.2 and FAQ #8209). However, this is because both sources of Shield are applying to the unit, not because Svellsongur is duplicating the Block effect.