If my 75 year old father has different properties, some with a great deal of land, what is the best way for 3 siblings to inherit/protect said land (Trust, LLC, etc.)?

An LLC is more for corporations.... not for estate planning.  You would only need an LLC if your father is in a business and needs to protect the assets from a claim by a third party.
If your dad wants to make sure that the land goes to a particular son or relative, then he needs to engage in some estate planning. Estate planning can be in the form of a will, a life estate, or direct transfers.  A will can make provisions for who will inherit what and how the taxes will be paid for any gifts (if applicable).  A life estate is a way to arrange for a transfer after your father's passing.  Basically, your father would retain the right to the property during his lifetime, but automatically pass to a survivor on his death.  The advantage is that it will pass automatically without the need for a probate of his estate.  He can transfer assets while he is still alive, but any recipients could potentially be taxed on any gifts. 
To really know what is the best option, your dad needs to visit with a probate and real estate attorney... each can look at his actual assets and help determine which is the best for his situation and those that he intends to gift.