#JavaScript (ES6), 73 54 50 bytes

(Saved some bytes thanks to @LeakyNun, the edited challenge, and @CraigAyre.)

s=>s.match(/../g).map((c,i)=>('0x'+c)/(i-3?1:255))

let f=

s=>s.match(/../g).map((c,i)=>('0x'+c)/(i-3?1:255))

console.log(f('00ff0080'));

JavaScript (ES6), 73 54 50 bytes

(Saved some bytes thanks to @LeakyNun, the edited challenge, and @CraigAyre.)

s=>s.match(/../g).map((c,i)=>('0x'+c)/(i-3?1:255))

let f=

s=>s.match(/../g).map((c,i)=>('0x'+c)/(i-3?1:255))

console.log(f('00ff0080'));

#JavaScript (ES6), 73 54 51 bytes

(Saved some bytes thanks to @LeakyNun, the edited challenge, and @CraigAyre.)

s=>s.match(/../g).map((c,i)=>eval('0x'+c)/(i-3?1:255))

let f=

s=>s.match(/../g).map((c,i)=>+('0x'+c)/(i-3?1:255))

console.log(f('00ff0080'));

#JavaScript (ES6), 73 54 50 bytes

(Saved some bytes thanks to @LeakyNun, the edited challenge, and @CraigAyre.)

s=>s.match(/../g).map((c,i)=>('0x'+c)/(i-3?1:255))

let f=

s=>s.match(/../g).map((c,i)=>('0x'+c)/(i-3?1:255))

console.log(f('00ff0080'));

#JavaScript (ES6), 73 54 bytes

(Saved some bytes thanks to @LeakyNun, the edited challenge, and @CraigAyre.)

s=>s.match(/../g).map((c,i)=>eval('0x'+c)/(i-3?1:255))

let f=

s=>s.match(/../g).map((c,i)=>eval('0x'+c)/(i-3?1:255))

console.log(f('00ff0080'));

#JavaScript (ES6), 73 54 51 bytes

(Saved some bytes thanks to @LeakyNun, the edited challenge, and @CraigAyre.)

s=>s.match(/../g).map((c,i)=>eval('0x'+c)/(i-3?1:255))

let f=

s=>s.match(/../g).map((c,i)=>+('0x'+c)/(i-3?1:255))

console.log(f('00ff0080'));