Centre-half Duffy is currently on loan with Championship side Yeovil, full-back Garbutt is with Colchester United, whilst Portuguese midfielder Francisco Junior is in Holland with Vitesse Arnhem.
"It's something that is vital for any player in a stage of their young career in order to fulfil your potential," said Martinez.
Garbutt has just extended his spell and Martinez said of him: "The team and the Colchester manager are really pleased with his progress and I've been really happy with the way he has been adapting to what he has had to do for the team.
"He's playing a really important role with Colchester. He has already got five yellow cards so that shows you he has been getting really stuck into his defensive duties.
"He's been successful there and they're pleased with him so there's no reason not to extend."
He is also pleased to see Duffy being tested in the second tier, adding: "The Championship is exactly what Shane needed. He's been performing at a real high standard."
And whilst Junior has made just a couple of league appearances, Martinez still feels he is gaining good experience.
"Junior has had a difficult start at Vitesse," he said.
"It's been difficult for him to have a big role there. They were in a very good position and performing really well, so it's been hard for him to get playing minutes.
"From that point of view, he has been the one that probably hasn't worked out as well as we expected or wanted.
"But there's still a long time - it's been a good experience for him in a different environment and he's been working hard to be available and important for the team.
"That experience in itself is going to help him from a human point of view and hopefully from now until the end of the season it's going to help in his football side."