How to Reply on Legal Notice from 3BHK owners to follow flat Maintenance charge

We are part of a housing society in Bangalore. The society is having total 196 flat with various size. 55 are 3BHK and rest are 2 and 2.5 BHK. The 2 and 2.5 BHK are with area range of 1068 sqft to 1150 sqft. 3 BHk on other hand are from 1600 to 1700 sqft. While purchasing the Flat it was told to all owners that maintenance will be as per area of the flat. The builder was maintaining the society for first 1 year and had collected maintenance as 2.5 rs per sq ft. After the society was formed it was collected 2.75 per sq ft Now after 1 year of association formation few 3BHK owners has opposed the per sq ft model. Now 2 and 2.5 BHK owners and 3 BHK owners had multiple discussion and arguments over the maintenance method. Then association collected all data points and conducted voting as per bye laws and decided to go with per sq ft model. Then 3BHK owners demanded Builders intervention and he has suggested one Hybrid model where all 2 and 2.5 BHK will share the additional maintenance. 2 and 2.5 BHK rejected this model and 3 BHK went wilfully defaulters by not paying maintenance as per sq ft. When association asked them to pay the maintenance as per association bye law they have sent a legal notice to the association. Now how as a group of 2 and 2.5 BHK owners we can reply to the legal notice. What are the legal options we have to claim our stands