a(3) = 292319 which is the concatenation of 29,23 and 19.
MATHEMATICA
Table[SelectFirst[FromDigits[Flatten[IntegerDigits/@Reverse[#]]]&/@ Partition[ Prime[Range[1000]], n, 1], PrimeQ], {n, 20}] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, May 30 2021 *)