According to Elixir guitar strings a set of medium electric strings in standard tuning on a 25.5" scale six string guitar will generate 118 lbs of pressure. Using the numbers provided on that same page you could estimate that a 3 string using the 3rd, 4th and 5th strings tuned to GDG would generate a touch under 60 lbs.
This is important to a builder because the structure needs to accomodate this load but to a player the extra pressure translates to higher tension on the string. Before looking at tuning and string selection you need to understand the impact of string tension on your playing.
| High Tension | Low Tension |
| Thicker string guage | Lighter string guage |
| Easier for slide | Easier to bend strings |
| Fuller tone | Lighter tone |
| Extra pressure on the neck | Too loose and they won't sustain |
| Harder to fret | Easier to fret |
How does this effect string selection?
Firstly it should be clear from the table above that if you're playing slide you'll go for heavier strings. If however we're using open tunings we occasionally want them to produce notes other than those they are intended to produce. Let's look at how this impacts on string tension.
String guage
The most obvious example is the common tuning of GDG using the 3rd, 4th and 5th strings from a standard set of guitar strings.
The 3rd and 4th strings are tuned the same as a standard guitar so the tension will match the pitch. The bottom string (lowest pitch) however is normally tuned to an A so bringing it down a tone to a G will reduce the tension on the string. You could compensate by using a slightly thicker string but in practice most people won't bother with the extra fuss and expense. With only a one tone drop you generally won't notice a significant reduction in tension on the thicker string.
It does illustrate how tuning impacts on string selection though, and GDG is a very common tuning. An extra-light set for example will produce much less tension and reducing it even further by tuning down may be too much, particularly on a shorter scale guitar. You also want to keep the tension around the same on all your strings so if you're low string feels too loose maybe you do need a thicker one.
Scale length
Shorter scale guitars require less tension on the strings to get to the same pitch as a longer scale guitar. If yours is significantly shorter than 25" you may want to go for even thicker strings if you're after a higher tension.