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The first difference is that Splash 1 and Splash 3 are mutations of the MITF gene and Splash 2 is a mutation of the PAX3 gene so two different genes are being affected. Both of these genes control important parts of a horse's body, so severe mutations will interfere with that work as well as producing white markings. This is why some combinations are lethal.

Here are some examples of horses with the various Splash mutations, no other white patterns except maybe Roan, and Medium White Factor for comparison. Note that these are single examples and that there are many variations in results given by the search parameters.

Splash 1 is probably the least damaging of the three and produces less white with the same amount of white factor. It is non-lethal in combination with either of the other two splash mutations and with itself, therefore is a completely safe gene.

Heterozygous Splash 1, Medium White Factor

http://www.huntandjump.com/horse.php?horseid=3164647

Homozygous Splash 1, Medium White Factor

http://www.huntandjump.com/horse.php?horseid=3163893

Splash 3 is much more aggressive, and breaks the MITF gene more severely, although I speculate that it causes damage in a different place or places than Splash 1, since the two can be safely combined. However, it does so much damage that it is homozygous lethal and cannot be combined with Splash 2 either.

Heterozygous Splash 3, Medium White Factor

http://www.huntandjump.com/horse.php?horseid=3037103

Splash 2, as noted above, breaks a different gene. It causes severe enough damage that it is homozygous lethal and lethal with Splash 3. It produces an amount of white that falls between that of Splash 1 and Splash 2

Heterozygous Splash 2, Medium White Factor

http://www.huntandjump.com/horse.php?horseid=3410173