DDR3 is pretty slow for ETH mining, look at the GDDR5X it's slower than GDDR5 and DDR3 is way slower for mining, but it would be probably cost effective with those DDR3, seems like DDR4 are too expensive that's plus.
DDR3 has one advantage over newer memory architecture....it handles small chunks much better then DDR4/5
DDR3 memory is faster than GDDR5 memory when you only want to fetch a tiny chunk. GDDR is designed to access huge chunks with high throughput. DDR4 vs GDDR5 for a 128 bit fetch is no-contest DDR4 winner. In addition to that, FPGAs now support OpenCL, with a properly designed FPGA setup, a miner with an OpenCL shouldnt even notice its running on a FPGA
https://www.vijaypradeep.com/blog/2017-04-28-ethereums-memory-hardness-explained/