Hello.

I can tel you this :

~ Your group needs a minimum of 27 participants in order to unlock all 6 battlefields.
~ If none of the warring groups reached 27 participants in last 3 weeks, they will have to war within the 3 top fields (10, 8 and 7).
~ It is possible to fight as group with only 3 battlefields against group with 6 battlefields. However, if small group with only 3 battlefields is fighting group with 6 battlefields, both of the groups will have 6 battlefields. As long as one of the fighting groups has met this requirement (minimum of 27 participants), both groups will have 6 battlefields.
~ Is it possible to have 6 fields on Wednesday but only 3 fields on Saturday ? The amount of battlefields is calculated according to group's last 3 week participation. So no, it is not possible, not in general situation.