We will assume that the sequence is geometric:

a1 = 3

a2 = a1*r, where r is the common ratio

6 = 3*r

r = 6/3

r = 2

Indeed for 3rd term a3 we have:

a3 = a1*r^2

a3 = 3*2^2

a3 = 12

Thus for the n th term we have:

an = a1*r^(n - 1)

for the 10th term we have:

a10 = a1*r^(10 - 1)

a10 = a1*r^9

a10 = 3*2^9

a10 = 1,536

the 10th term of the sequence is 1,536.