There are a number of ways you can cause "things" to appear using a (live:) macro, which you use can depend on what exactly that "thing" is. And your question is a little vague on exactly what "things" you are talking about in this case.
I will assume you are trying to make some hidden text appear after 3 seconds, although this same technique can be used to reveal other "things".
|hidden)[Some hidden text
Some more text
... the above example uses the following:
a. Hidden (named) Hook Markup to initially hide the relevant text.
b. A (live:) macro with a 3s (3 second) delay.
c. A (show:) macro to reveal the contents of the relevant named hook.
d. A (stop:) macro to cause the execution of the live macro's associated hook to occur only a single time.
note: You can change the name of the Hidden Hook to something other than the hidden I used as long as you remember to also change the hook name passed to the related (show:) macro.