Ask NextSet for a classic one-muscle-a-day split and tell it you train three days a week, and it won't give you one. It builds you something else and says so on your plan screen, in as many words.
People find that annoying. Here's the arithmetic.
The maths nobody does
A classic split has five or six sessions: chest, back, shoulders and arms, legs, maybe a glutes-and-core day. If you run that at three days a week, you don't train chest twice a week. You train it once every ten to twelve days.
Count it out. Week one: chest, back, shoulders. Week two: legs, glutes, chest. Your chest sessions are eleven days apart. Your back sessions are eleven days apart. Everything is.
That's not a training frequency. That's a rotation you happen to be in.
Why it matters more than the split you picked
Every muscle you train gets a bump in protein synthesis that runs for roughly one to two days. Then it's over, and you're just waiting. Eleven days between chest sessions means nine of them are doing nothing for your chest.
This is the single most common reason people stall in a gym: not the programme, not the exercises, not the technique. Frequency. They picked a split designed for people who train six days a week, ran it three days a week, and then concluded that lifting doesn't work for them.
What we do instead
Below five lifting days a week, NextSet gives you a split where every pattern comes round at least once every seven days. At two or three days that's full body. At four, upper/lower. At three it can also be push/pull/legs, because that still lands each pattern weekly.
The rule is one line in the engine: splitsFor(liftingDays). Below five, the classic split isn't in the list, so you can't be given one by accident.
And when you asked for one and didn't get it, the plan tells you:
A classic one-muscle-a-day split needs 5+ lifting days — at 3 you'd train each muscle barely once a fortnight. We've built you a Full Body plan instead so everything gets hit every week.
The exception, and why it exists
If you go into the custom plan builder and construct a one-muscle-a-day split by hand at three days a week, we'll let you.
That isn't inconsistent. The generated plan is for someone answering a questionnaire who reasonably expects us to know better — refusing there is us doing our job. The builder is for a coach who has a reason. It'll still tell you what the volume looks like; it just won't stand in your way.
If you're going to fight the arithmetic, you should at least be able to see it.