I cannot answer definitely, but I did upgrade directly from sp3 to sp6awith no problems on some machines. However, I do not know if there is any issue if any apps on the machines are reliant on specific versions of any drivers in 3. That would create a problem and may require updates to those apps also for them to be useable afterwards.
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"When the debate is lost, slander becomes the tool of the loser." - Socrates
Vita contingit, Vive cum eo. (Life Happens, Live With it.)
"Life is not measured by the number of breaths we take, but by the moments that take our breath away." -- author unknown
"De omnibus dubitandum"