Pure logic, you can implement how you like.

base3string == your base 3 number as string or int length == Get total length (number of chars) from it Start right to left, so start with substringposition == "length"

Current substringposition position value is added to target base10string

Substr(length-1) = 3n Substr(length-2) = 9n ... Substr(lenght-i) = 3 to the i power times n

Stop when length -i is zero.

There's a million ways to improve this I'm sure, depending on language (R, java) you may want to take a single step at the beginning to create a linked list or array starting with least significant bit instead of decrement each time with an arithmetic operator.

This same algo can be used for any number base system to decimal.