Someone posted this on reddit, I thought it was a good read:
TL;DR: 6th+18th+20th+Filler could get us to #4, but based on precedent we'd need something else of real substance to get into the top 3.
I'll look at this two ways: precedent and analytics.
Precedent:
The 2018 Draft's Mavs-Hawks trade is a good precedent for moving up a couple of spots in the top 10.
Mavs received: 3rd pick (Luka)
Hawks received: 5th pick (Trae) and future first rounder (top-5 protected 2019, top-5 protected 2020, top-3 protected 2021, top-3 protected 2022, unprotected 2023 [ended up being #10 in 2019, Hawks picked Cam Reddish])
Vegas had the Mavs with the 9th lowest win-total (34.5) heading into the 2018-19 season, so let's say that to move up 2 spots, the Mavs had to offer a pick with an expected value of ~9.
So, what are our 18th+20th picks worth compared to the ~9th pick? The 2014 Draft's Denver-Chicago trade is good precedent for the worth of our 18th+20th picks.
Denver received: 16th (Nurkic) and 19th (Gary Harris), 2015 2nd rounder
Chicago received: 11th (McDermott), Anthony Randolph
So 18th and 20th would be around the ~11th pick value, less than the ~9th pick value of that Mavs pick. So in order to move up 3 spots to #3, we probably have to add something of real value other than 6+18+20 (the Bulls pick... Keldon?).
Analytics:
Kevin Pelton of ESPN and Jacob Goldstein (now works for the Wizards) both formulated values for each pick of the NBA Draft based on historical data. Interesting, both of them value 6+18+20 as better value than that of the #2 pick: see chart.
I can't imagine any team that made it into the top 2 actually trading down though for 6+18+20. It seems like a huge PR hit to land a top-2 pick and trade down all the way to #6, and I doubt many fan bases (or ownership groups) would be very excited about the 18th and 20th picks when they could have had a hyped #2 pick. Moving 3 spots up to #3, however, might be more feasible based on the precedent above.
Overall Thoughts:
In order to trade up, we would need a situation similar to the Luka-Trae trade. The Hawks evaluated Trae and Luka the same, so they felt fine trading down and getting an estimated value of the 9th pick next year.
Orlando might be a good trade partner if Jonathan Isaac could actually stay healthy. The consensus top 3, Jabari/Chet/Paolo, are all big men, and a guard like Jaden Ivey or Johnny Davis might be a better fit for the Magic. So something like 6, 18, 20, and the Bulls pick might be enough.
I do think that this year's trade deadline shows the Spurs really do want to trade up though, and are gathering picks to do so. I believe this season has the most in-season trades in franchise history so there does seem to be a real sense of urgency. Plus, it's hard to see us trying to develop 3 first-rounders next year while we still have all these guys who need playing time like Murray, Jakob, Keldon, Vassell, and Primo. I suppose we could just draft 3 first-rounders and then do our usual G-League development, but then at some point the minutes in the big squad would get really hard to come by.