This is the second time I have lost the pin for the lift arm on the 3ph. I found it the first time but this time it is lost in the snow so I had to make another one. The original clips are very small and I found they fall out easily so I drilled out the holes and install bigger clips.
Good idea with the cotter pins-- those snap-in pins will pop right out if something hits them or catches on them just right. As low as those 3PH arms are, that sounds like what's happening. irvingj