A friend of the family told me that her husband had set up a 3-part trust fund in 1979. Part A went to a college, Part B was for her, and Part C was to go to me. He passed away shortly thereafter. She has now passed away late last year. Her trust officer insists she knows nothing about a trust for me and that my friend would have had rights to change anything about all parts of the trust. What if there is still a trust for me and no one knows about it at the bank where the trust officer works? Aside from being able to use the funds, it would be a shame to think it was just sitting somewhere abandoned.
You really are flying blind unless you can track down a copy of the trust agreement(s). From what the trust officer says, it sounds like Part B at least was part of a revocable living trust that remained revocable and amendable by her following his death.
You might try to find out which attorney worked with him back in 1979 and see who has the files. Or, if her estate is being probated/administered, perhaps you can inquire about what she meant.
The only other suggestion is that if a separate trust was established for you and was "abandoned" whoever held the funds might have delivered them to the state as unclaimed property. I don't know what the procedure is in TX, but here in WA we go online and can search to find out whether they are holding any unclaimed property for us. Of course, eventually the unclaimed property escheats to the state.
Unless you have a pending lawsuit, it will be difficult for you to compel anyone that is not a confirmed trustee to tell you anything. You could hire an attorney to conduct pre-lawsuit discovery (this can be expensive). In short, if the trust officer will not provide you with information that you find satisfactory, you may need to file a lawsuit to obtain the information. On another note, you might check with the county clerk's office in the county where your friend's husband passed away. If his estate were probated, there would have been an inventory filed. The inventory would contain a list of his assets. Trust assets are not required to be listed in the estate's inventory, but you might learn something by what property is not listed (i.e., if it wasn't estate property, it could have been trust property). Short of filing suit, you and / or your attorney will have to be creative to find the answer. There are a number of new questions that are presented by your post that should be discussed in detail with your attorney.
You also mentioned that the trust officer told you the Part B beneficiary could have changed the trust's terms. While this is perhaps conceivable in some circumstances, using the brief facts you present it seems unlikely.