First, you'll need to edit your UTWeapon.ini file, located in My Documents\My Games\Unreal Tournament 3\UTGame\Config

You will see entries for each weapon. You can simply change the first option you see (UTGame.UTWeapon) and it should give you the same crosshair for each weapon. If not, you can find

[UTGame.UTWeap_InstagibRifle]

and

[UTGame.UTWeap_ShockRifle]

Create a new line by pressing Enter after the last square bracket and have the code look like below. You will have to type in the new lines.

[UTGame.UTWeapon]
bUseCustomCoordinates=true
CustomCrosshairCoordinates=(U=474,V=27,UL=9,VL=9)
CrosshairColor=(R=128,G=255,B=0,A=258)

To note, you will need to enable Simple crosshairs in your Settings menu. By default, this is set to Normal. Choosing Simple will allow the custom crosshairs to be used.

The CrosshairColor can be whatever RGB value you want. Google for RGB colors and when you have the color you want, simply insert those numbers into the R= G= B= The A is for Alpha in the event you want it semi transparent. The color I have is a bright, neon green.

The rest is taken from Epic forums:

[UTGame.UTWeapon]
CrosshairColor=(R=255,G=255,B=255,A=255)
bUseCustomCoordinates=true

dot large
CustomCrosshairCoordinates=(U=474,V=27,UL=9,VL=9)

Dot:size med
CustomCrosshairCoordinates=(U=282,V=92,UL=8,VL=8)

Dot:small
CustomCrosshairCoordinates=(U=276,V=84,UL=2,VL=25)

square:
CustomCrosshairCoordinates=(U=35,V=334,UL=5,VL=5)

cross with a dot
CustomCrosshairCoordinates=(U=278,V=86,UL=18,VL=22 )

Pointer + Dot
CustomCrosshairCoordinates=(U=270,V=96,UL=34,VL=2)

Circle
bUseCustomCoordinates=true
CustomCrosshairCoordinates=(U=380,V=320,UL=26,VL=2 6)

Stinger Rectangles
bUseCustomCoordinates=true
CustomCrosshairCoordinates=(U=451,V=25,UL=57,VL=16 )

Open X
bUseCustomCoordinates=true
CustomCrosshairCoordinates=(U=406,V=277,UL=20,VL=2 0)

Shock Pointer
bUseCustomCoordinates=true
CustomCrosshairCoordinates=(U=261,V=25,UL=56,VL=14 )

Phoenix
bUseCustomCoordinates=true
CustomCrosshairCoordinates=(U=18,V=106,UL=22,VL=26 )

Pointer + Dot fat_boy
CustomCrosshairCoordinates=(U=408,V=26,UL=16,VL=14 )

Circle 2
CustomCrosshairCoordinates=(U=406,V=83,UL=22,VL=23 )

Half Circle
CustomCrosshairCoordinates=(U=145,V=404,UL=23,VL=7 )

small square
CustomCrosshairCoordinates=(U=155,V=414,UL=4,VL=4)

Half Circle 2
CustomCrosshairCoordinates=(U=380,V=332,UL=27,VL=1 3)

UT Dot xsmall
CustomCrosshairCoordinates=(U=294,V=225,UL=5,VL=4)

Phoenix 2
CustomCrosshairCoordinates=(U=398,V=40,UL=45,VL=9)