USMC,
Yes you will get more depth in single frequency mode. In single frequency mode all of the transmit power goes to one frequency. You gain a little depth but you lose the additional target analysis info you get when using 3 frequencies. If you are in a trashy park and most targets are less than 6 inches use 3 frequency and you'll dig less trash. If you are at an old site without much trash and looking for targets 6 inches and deeper, then use single frequency.
In theory 2.5 is best for silver, 7.5 is best for brass, and 22.5 is best for gold. However in practice it's a little more complicated. 22.5 is best for small targets and will often be your best choice for silver coins and other small targets. Also, you may find places where the EMI is so bad you have to choose the single frequency with the least noise.
Also due to mineralization and other soil conditions sometimes a particular frequency will work better in one place than another.
I suggest bringing a couple sample targets to a new site and burying them at 4-6 inches to see which frequency responds best. There is lot's of good info regarding single frequency use in a post from RC Snake on another forum. You may also want to look at the the Advanced Users Guide from Carl at Whites.
Good luck!
Three Ringer