On a side note: the attack bonus (from the hero or siege) is applied directly on each type; in other words, it does not affect the mounted/unmounted ratio (be your hero on foot, on horseback or flying on Drogon)
The attack bonus is added to the hero power, thus, if the is mounted, you get +3*swordsmen mounted attack
For a simple experiment run the ingame calculator with hero power 0 and whatever set of attack/defense (but with at least some troops of the weapon type), you will notice the defense value changes if you put your hero on horseback or not (because of the now different proportion of mounted/unmounted attacking value)
a follow-up question:
Provided that I have ONE open slot and I am atacking a regular village with three chiefs, all three will lower the loyalty, correct?
Do I need to have open culture slots (i.e. slots to found or conquer a city) to help a friend lower an enemy village loyalty?
If so, in the case of a city, do i need two open slots?