The Blessed Power of Three is and will be a strong theme throughout the series run of Blessed, as the three main characters, the Blessed Ones, are connected through the Blessed Power of Three. This is said to be the strongest kind of magic that would ever exist in this world, surpassing that of the Charmed Ones (and the Power of Three).
The Triquetra is the thematic symbol of Blessed, which specifically represents the Blessed Ones, themselves, along with their Blessed Power of Three, and which is marked on the Halliwell Book of Shadows. This applies to the Charmed Ones as well.
While most of the spells written in the book require the Power of Three to take enough effect, its very possible that two Blessed Ones could enact the spells to work or even just one Blessed One. It won't always be necessary for all three friends to chant a spell together as long as they are all together at the same time.