Cash poor asset rich - best way to get mortgage

I'm hoping to get financial advice. In summary we are asset rich but cash poor and need to reorganise our finances so that we can raise some capital to purchase an apartment

Our portfolio
House approx value £240k no mortgage
Rental prop - commercial office value £220k, annual rent 13k
Rental prop 2 - commercial office value c£130k, rent 6k per year

These properties are owned by my father and I. How can we use the equity tied up in these properties to guarantee/fund a new mortgage?

We need to raise some cash to raise a deposit to buy a flat, we are open to selling property 3 any additional advice would be much appreciated.