Three dots are used to indicate omitted values or terms, but the ellipsis character (…; unicode 2026) will usually not have enough space between the dots. Instead, use regular full stops separated by spaces.
In many cases, centred dots are preferable. As with lowered dots, to space them correctly you may need to use 3 single centred dots (·; unicode 00b7) rather than the midline ellipsis character (⋯; unicode 22ef). Use centred dots if the surrounding characters are vertically centred and lowered dots if the surrounding characters are lowered:
x1 + x2 + · · · + xn not x1 + x2 + . . . + xn
x1, x2, . . . , xn not x1, x2, · · ·, xn
When using 3 dots in the middle of an expression, the characters on either side should generally be the operator or separator that is repeated:
x1 + x2 + · · · + xn not x1 + x2 + x3 · · · xn
x1, x2, . . . , xn not x1, x2. . . xn
