We must be leaving ,,,,,,

among must, may and should,
must means certain
e.g. You must clean up your room now!
must can be used in orders also

may is comparatively not that certain
e.g. I may get wet if I don't bring an umbrella.
may can be used in predictions also

should is commonly used in advice
e.g. You should go to bed earlier.
Therefore, it is relatively neutral, it does not contain meanings of certain or uncertain

and among them,must is less polite
using -ing(present continuous tense) is for personal plans
and this is more informal
using simple present tense is for organized events and schedule
and this is more formal
hope this could help you=)

We ( must / may / should ) be leaving soon , (可能會走)
We ( must / may / should ) leave soon (一定會走)

個degree吾同

即係個語氣既強烈程度吾同

May係100%sure
must 50%
should 10%

個possibility 可能性(較大/較小)之分