I was just wondering if abilities/effects triggered by kneeling condition apply also to kneeling a card to defend/attack in the challenge phase?

e.g. Margaery Tyrell gives +3 str upon kneeling but does it have to be initiated seperately or can it be a by-product of defending a challenge for example?

It depends. When the text reads "Kneel (this card) to (X)", then you kneel it for that purpose alone (and make it unusable for any attack or defense later). When the text reads "When (this card) is kneeled, you gain (X)", or something similar, the effect is addtional, so when you kneel it to atttack, you gain the effect, too.

makes perfect sense, thanx