Ask your Strata Manager for a copy of the Strata Roll, to which you are entitled, even if they charge you the small inspection fee to do so.
Then check if the “tenant” renting the car park is listed on the roll per https://www.fairtrading.nsw.gov.au/housing-and-property/strata-and-community-living/strata-schemes/responsibilities-of-the-owners-corporation
If not, lot 3 would be in breach of the tenancy notice being given within 14 days: http://www5.austlii.edu.au/au/legis/nsw/consol_act/ssma2015242/s258.html
Another issue may be that lot 3 are breaking the Development Application (DA) conditions for your complex. Depending upon the age of your block the DA would have a specified number of parking spaces based upon number of bedrooms, and this would normally be related to what the council required under their planning guidelines. If a lot then “commercially” leases their garage to someone outside the complex, they are going against the DA.
Re your issue of the common area full of items – take lots of photos. Without this it can later on become a he said, she said situation.