After the first three numbers, each number is the sum of the previous three numbers. So the next number is $$11 + 20 + 37 = 68$$. 
What is the next number in this sequence?  $$1$$, $$2$$, $$3$$, $$6$$, $$11$$, $$20$$, $$37$$,~\uline{~~~~~~~~~~}~? ． 
[1] $$47$$ 
[2] $$54$$ 
[3] $$57$$ 
[4] $$68$$ 
[5] $$74$$ 