Firstly welcome to the forum and I hope you enjoy your time with us.

Not a bad opening question so here goes with an answer or two 

Is it easy getting permit to build units in the first place. It is if the property is zoned for such and the land size allows for 3 units to be constructed.

What would the approx cost be to build all 3 excluding the land. Bit like saying how long is a piece of string becuae it will depend on the size, quality of fittings etc. Single storey lowset or garages below and units above? Unit, Townhouse or 3 x single residential dwelling.  There are so many variable it is impossible to give you a figure.

Is it easy getting loan with 3 parties together. Not ideal but if you all 3 want to be involved then i guess there isnt much choice. Wont worry at this stage about what entity you setup to buy the land and do the construction but bear in mind with 3 of you having ownership to the Title all 3 of you will be on the loan application. All of your liabilities will need to be taken into consideration when assessing the serviceability.

Would suggest a trust structure if you intend to buy, build and onsell.

Is it easy to sell the units and sub-divide separatly once built. Would normally be condition of the Development approval. Course you will need to decide if you make them Strata Title or can get them to be Freehold. A good local Town planner should be able to help you further.