after you switch to trever and and do some missions you will be able to switch back to the other two and you will have the funds to buy propertys, you unlock propertys when you switch to trever but you dont have the money to buy them. go to your map and select the second icon down and you can cycle between all the propertys, you need to reveal the fog of war to find all of them
Properties or businesses? Because when I unlocked Trevor and after the first plane mission I was able to purchase businesses. I haven't seen anything on properties.