Which of the following are geometric series?

1. $1,\:6,\:11,\:16,\:21,\:26,\:\dots$
2. $9,\:6,\:3,\:0,\:-3,\:-6,\:\dots$
3. $1,\:3,\:9,\:27,\:81,\:\dots$
4. $4,\:-8,\:16,\:-32,\:64,\:\dots$
1. $P$ and $Q$ only
2. $R$ and $S$ only
3. $Q$ and $S$ only
4. $P, Q$ and $R$ only