Hello and welcome to the forum!
There are several reasons why fewer than three journeys are sometimes returned. This could be because other journeys are significantly longer than the fastest route or because they require many changes. To keep the system efficient, it will sometimes not return such journeys and we therefore cannot guarantee that three journeys will always be returned.
If you have specific examples of journey requests that produce an unusually small number of results, we can take a look at them.
One thing you might like to try is to use the Alternative Routes feature, which may return additional results: