I wouldn’t add a third lift that you’re worrying about percentages on, because your two days of barbell lifting already have enough work, both physically and mentally.
Suggestion 1: superset DB goblet squats with your OHP on Thursday. These wouldn’t be done as a percentage, but they add up. Get in 3-5 sets of 10 with an 85 lb DB and that extra work will make a difference.
Suggestion 2: Add a “circuit” on another non-531 day, similar to Jim’s Walrus challenge. Here, you do a circuit of push ups, pull ups, and squats with a weight vest. If you don’t have a weight vest, at least use DB squats. You’d do five rounds, and if each round has 10-20 squats, again these will add up. The entire “challenge” only takes 15-20 minutes.