It would depend largely on which c3 (samuel/sam2/ezra/Nehemiah) we're talking about, but in general none of them would compete with intel or amd offerings at the same clock.
when we comparing the performance per mhz rate, that was bad, okay, but that does not means anything. the power consumption, the performance, and the price is the deciding factors.
600 mhz via c3 dissipated typical 7w under load, with 13w maximum peak, and 1w in idle.
600 mhz amd k7 continously 50w (they decrased it a bit in the second version, however, to some 40w, no power throttling)
600 mhz intel pentium 3 - 42w (later models decrased the power consumption seriously, however, still no power throttling for idle)
Via C3 offered the same performance/watt rate with the earlyer amd and intel competition (+/-5%) - while under typical tasks, it had 10x-20x smaller power demand, offering superior solution for notebooks, laptops, or even some desktop computers, where power demand and/or silent/fanless operation was needed. with c3, via owned these market segments, amd and intel was unable to competite with via in these segments, this resulted a slow market share incrase for via, and also, under the same watt rate, it performed the same maximum performance.
via c3 was an awesome solution in every mentionable ways, and it was much cheaper than the competitors later-released low-power k7 and p3 chips. i didnt seen anybody to complain about them until yet. of course, competitors also had the newer low-power chips on the market when the c3 is released, but a 20w p3 was like... 600-1000 usd without even power throttling...
so in short, while c3 ate few watts, it was cheap, and was relatively fast for its power consumption. (the situation can be compared to the intel atom chips of the 2010's)
(however C7 was a more brutal successor, sometimes reaching pentium4 on the same clock, while eating 5x-50x few watts, catalizing the market share of via)