write a function inside the summation in a more condensed form

Dear R Experts
that's my original equation

x=c(2,4)

Y1=sum(sapply(1:2,function(i){sum(sapply(1:i,function(j){(3^(x[i]+x[j]))}))})) 
y1 

[1] 7371 
 I want to write the inside function (3^(x[i]+x[j])) in a more condensed form cause this will help me when the multiple summations are more than two 

I've tried the following form

y2=sum(sapply(1:2,function(i){sum(sapply(1:i,function(j){(3^(sum(sapply(i:j,function(l){x[l]}))))}))})) 
y2 

[1] 819 


But it didn't work, it gave another solution 
Any help or recommendations