If you do a 3A in the combo, the solo Axel has to be a double.
You can do a quad out of steps. Goebel did one in 02, Yagudin did it before it was allowed as part of the jump combo, Elvis did them....
Goebel did a 4s-3t in the SP in SLC along with 3a and 3f.
In the 99- 2000 season, there were some top guys doing solo quads in the SP plus 3a-3t or 2t and a 2a, usually Yagudin and Plushenko, sometimes Weiss, Goebel and Li.
Yagudin 2000 Worlds
http://www.youtube.com/watch?v=tyoSJKjZums
Weiss 2000 Worlds
http://www.youtube.com/watch?v=9tCw7H1WPi4
Goebel 2000 Worlds
http://www.youtube.com/watch?v=eHXJSMaa3S4
Following 2000 Worlds, quad combos were allowed in the SP. This was the standard for the top guys for the next few years while the Russians were dominant.
Yagudin 2001 Worlds
http://www.youtube.com/watch?v=og59QN4L_ko&feature=related
Other guys were doing the 3a, 3-3 and solo triple layout most of the time throughout that cycle and in fact up to today. Interestingly, Yagudin and Plushenko both did the old 3a-3t, 3z, 2a layout in the 98-99 season when they could have done another triple.
Yagudin 1999 Worlds
http://www.youtube.com/watch?v=NaRu4vWzfhY
Plushenko 1999 Worlds
http://www.youtube.com/watch?v=-jwDFjxfgq4
After the 88 Olympics, there were a couple of years when the 3a was allowed to be repeated in the SP. The ISU had just allowed the solo jump to be a triple instead of a double. (A real explosion of technical difficulty after years of seeing mandatory double flips or double loops as the solo jump) So folks like Browning, Petrenko, and Eldredge ran with the new freedom and were doing 3a-3t or 2t or 2 lp, 3a from steps and 2a. For obvious reasons, the ISU thought too many axels were being done and required the solo jump to be of a different type. That led to the longtime layout of (usually) 3a-3t or 2t, solo triple (usually lutz) and a 2a.
Browning 1989 Worlds
http://www.youtube.com/watch?v=JVDn8QfoLSw
Petrenko 1990 Worlds
http://www.youtube.com/watch?v=QimoBJwAdbE
Eldredge 1990 Worlds
http://www.youtube.com/watch?v=lxxMq12UNKo
So in reality, there was a two year stretch (88-90) when the SP was actually more difficult than the rest of the decade until 1998-1999.