Just a tip to ask to specialists in bachido. How do you replace the strings ? One by one, or three at the same time ? On a guitar we usually change the complete set because the strings oxidize at the same time and to preserve the unity of the sound between them. But what for the shamisen ? Generally when the san no ito is about to be changed, the ichi no ito is still in very good condition…
I’m going to necro 
this because I have a question to add. 
To the original question, based solely on my observation of how strings are sold—by type (ichi, ni, san) and material (silk, nylon, tetron, etc.) and in different counts (1/3/5), it would seem that it’s normal to replace one string at a time.
My question: when do you replace them? When they break is one obvious answer, but are there other reasons to refresh the strings? I’ve played almost daily for over two months and not a single string has broken. What I do see are indentations or stress marks on the strings and there has been a small, detectable degradation of tone. Should I keep waiting for them to break? Or is 2-3 months time for a fresh set?
It really depends on what you want out of the string, and the same advice goes for most stringed instruments.
For the best tone, many performers will change strings before performances, with enough time to break them in and have the tuning set. I’d say if you can hear a degradation in tone, then that would be the time to swap them out if you were going to be playing for an event, etc.
For practice purposes, I leave mine on until they break. Shamisen strings aren’t easy to come by outside of Japan, so I try to make my supply last as long as possible. If I can manage it, I’ll even let the string out and see if it’s long enough to re-tie and keep playing. The only reason I’ve switched them out before they’ve snapped is to change up the gauge or material of the strings.
If you’re lucky enough to be where you can run to the local shop and pick up new strings, or if you order in enough bulk to manage it, then you can change them whenever your tone starts to degrade, if you want to. If not, or if you’re just putting in a lot practice, there’s no harm in going until the strings give out on you.
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