Are there smaller chain rings available for Shimano Tiagra Triple?

Do you know what BCD is used for the smallest ring? 4 bolt holes could be 65mm or some other size.

https://cdn.shopify.com/s/files/1/0230/9291/files/Measure_BCD_5ad6ccfa-f146-4048-ac0f-e2279602c9e1.pdf is one way to measure it

Assuming your crankset is a FC-4703 then the specs are at https://bike.shimano.com/en-EU/product/component/tiagra-4700/FC-4703.html and suggest its 74mm BCD.

Sheldon says that a 74mm BCD will take a 24 tooth chainring, or 26 and 28 options as well. He also suggests a "deflector" to help the chain make the big jump down from the 39 without missing, to prevent damage to the frame.

Unfortunately the asymmetric four bolt pattern that Shimano switched to for these cranks is a new patttern and so far there are no small inner rings available. The best advice I can give is to switch to the older five bolt style crank, for which many small rings are available. I use 26-39-50 on the previous generation of Tiagra with no trouble.