I don’t quite understand the drawing. It looks like the common property area (that little rectangle at the top of the diagram) fronts onto the street. If so, can you put in a locked gate that can only be accessed from the street by the various utility providers (as well as lot owners)?
If I understand correctly, the meters and such are in each of the back gardens, so not all clustered together but spread along the back of the building in each of the different backyards. If that’s the case, you’d need a series of gates leading from one garden to the next.
I just looked up the Victorian strata legislation and it says you have to give 7 days notice to the occupier of a lot if you want to enter the lot to carry out maintenance, repairs etc (unless you agree a lesser time, or it’s an emergency.) The lot owner must provide access. Have a look at pages 35-36 of this document: 06-69aa020-authorised.pdf (legislation.vic.gov.au).
So I guess you have two options. One is to instal gates that ensure access without dealing with the lot 3 owner. You can do this with money and a special resolution if you can get enough votes. The other option, if lot 3 owner is being difficult, is to formalise what’s in the legislation, with specific reference to the layout of services in your own building, in a special by-law. The legislation should be enough, but a by-law means should make it very clear to the current and future owners of lot 3 (and any other recalcitrant lot in the future) that they have an obligation. And you don’t then need to be arguing what the legislation means – it will be spelled out more clearly in your by-law.