But with M3, you have to invest in fuel plus a flame engineer squad (170MP + 60 munitions) or 300MP penals. Also, Kubel can cap and constantly harass the map and cut offs especially in 1 v 1s. Kubel got a nice armor buff few patches ago too.
There's no point in directly comparing the M3 to the Kübelwagen like that. We need to compare their cost efficiencies. The M3 is more expensive, but it also has a huge impact on the game (against OKW). The Kübelwagen struggles to be worth the 210 it costs. M3 gets built in most Soviet games against OKW, while the Kübelwagen gets used quite rarely.
As people have said many times already: if the Kübelwagen were fine, players would use it. They don't. In almost all cases, they decide to wait for 40 mp and get Volksgrenadiers, because they are more versatile, more cost effective, more combat effective, and stay relevant throughout the game. Getting volks is simply a no-brainer decision right now.