If we ignore the 1, and look at the next three terms,
we can see that they form a geometric series with
common ratio 4.  But the first term would have to be
3/4, not 1.  It would be easy to find a formula for

3/4, 3, 12, 48 

an = ceiling(3*4^(n-2))

But hey!, we can make 3/4 become 1 by rounding it up to the
next integer.  Since all the other terms are integers,
we can simply use the the ceiling or "round up" function, as
it does not affect integers.  The ceiling function is usually 
denoted by a square bracket with the bottom part missing.  
I'll just indicate it on here by "ceiling( )"

So a formula is

an = ceiling(3*4^(n-2))

Edwin