How can i take my deceased dad’s property if he still has a small mortgage on it?

If there is no will, there are no executors--executors are appointed by a will. If there is a will, then whomever is appointed by the will as executor is the executor; that person must distribute the property as instructed by the will--i.e. if the will says it goes to the 3 of you, it goes to the three of you. But he will need to get legal authority from the probate or surrogates court to act as executor--it will be easy to get, if he's named in the will, but he still needs to apply for it (this authorityis often called "letters testamentary"); he then needs to contact the mortgagee/lender and find out whether they will let the people inheriting the house assume or take over the mortgage; or whether they can refinance it with a new mortgage; or whether the whole remaining balance needs to be paid in full--when your father passed away, his debts became due, and the lender can insist on payment in full immediately if they want, and foreclose if it is not made. In one way or another, you have to pay the mortgage and satisfy the lender.
If there is no will, then if the three of you are the only children and there is no surviving wife, the property will go to you. The process is similar in principal, but some of the paperwork and procedures are different. In this case, you'll need to apply to the court for one of you to be appointed the administrator or personal representative of the estate, which gives you executor-like power, then you contact the lender, etc.
There are also requirements for notifying your dad's creditors of his death, so they can put a claim in to the estate--basically, his debts become debts of the estate and must be satisfied before the heirs get anything.
It is a complicated process, and a lawyer is a good idea--I'm an attorney, and I know alot about this process, but because I don't do it day-in and day-out, and especially don't do it in FL (where my parents lived), when my father died, I hired a local probate attorney to do this for me, rather then trying to learn all the nitty gritty paperwork details myself.
If you are determined to do this yourself, you can find the information you need from the court clerk's office and the internet, but it is very detailed--think about getting an attorney.