The UA view of the reservation is different from the UA view of the ticket. It's OK for the UA view of the reservation to have some slightly 'old' flights on partners.

If you are able to check in online for segment 3, then this indicates UA knows you flew segments 1&2 (the coupon status of the relevant sectors is shown as "Flown" on UA's view of the eticket). If you are UNABLE to check in online for segment 3, and if you get an error message about a ticket / reservation mismatch, then you may have a problem.

You can view the coupon status for United ticket stock (016 tickets) by plugging in your e-ticket # and last name to 'manage my bookings' at saudiairlines.com. Check what is printed next to each segment for the segments you think you've flown -- if "Flown", you're ok. If "Checked In" or "Open For Use" or "Airport Control", you may have a problem.

For the next segment you're planning to check in on, the coupon status should be "Open For Use" or "Airport Control" until you have checked in (and everything previous should be Flown), and your coupon status should change to "Checked In" once you have done an OLCI (and if you undo your check-in it should revert back).

It's possible that this could all go wrong and a partner system could transmit no-show information to UA and cause UA to cancel the ticket, but it's very very unlikely. Plan for this worst-case outcome by keeping a printed copy of your eticket receipt and itinerary as well as a copy of your used paper boarding passes — but be aware that this kind of planning is pretty paranoid, kind of like keeping paper copies of all your bank statements to show the FDIC in case your bank suddenly goes belly-up and loses its electronic record of how much was in your account.