The alt is mechman 250amp adjustable voltage and the battery is xspower d1400 14v. I know that there is no definite answer but do you guys think that this would be sufficient to power 3 amps? (saz 3000d a jl 500/5 and a jl e4300) Maybe make it two batteries? Better alt?(recommendation for 1)

To charge that 14v battery, you need to have your voltage above 15v... You can't use it as your one and only battery for the vehicle.