THIRD cousins are considered quite distant in the gene pool. I once very much wanted to someday marry my mother’s first cousin’s son (making him my second cousin). I was too young then to even be considering marriage, and he was aabout four years older than I and definitely ready to be seriously looking for a shidduch.
He got married three years later (he lived in E”Y and we faithfully corresponded during that time, but he really was ready to be married, and I was not). He DID consider the possibiity, but his mother O”H, whom I loved dearly and who loved me, felt it would not be wise, because there were children in the family who were not well, and who were the result of a marriage betwen distant cousins. So we went our separate ways, he married a wonderful woman, I and married a wonderful man, and I fondly remember those years.