Images appeared to show Drissa Traore, Brandon Ormonde-Ottewill and Jeremy Balmy taking the legal high, commonly known as laughing gas, via balloons.
Swindon suspended the trio on 7 April and, following a club investigation, a statement said: "Swindon Town now consider this matter to be closed.
"The three first-team players have been fined for their actions."
The statement added: "The trio acknowledge that their actions were wrong and would like to apologise to their team mates, the club's staff and the supporters.
"They are keen to move on from this issue and get back to training to help the team as quickly as possible. All three will be considered for selection once again this weekend."
Head coach Luke Williams told BBC Wiltshire: "They will be available (on Saturday) because they will have served their suspension time but the boys will have to earn their right to be back in the matchday squad.
"I should think there's a level of conditioning that they've now lost because they have not been training for a fortnight, so they'll have to be re-conditioned.
"They'll have to show in training every day that they're desperately trying to make up for what they've done."
Swindon host Chesterfield in League One on Saturday.

Peter Lloyd, 45, lives in a tent in the garden of an "uninhabitable" house, Cardiff Civil Justice Centre heard.
Mr Lloyd, who has electromagnetic hypersensitivity, said he was entitled to direct payments to employ a helper.
The council, which was ordered to pay Â£12,000 in costs, had claimed he was not capable of managing money.
Quashing the council's decision on Wednesday, Mr Justice Singh said electromagnetic hypersensitivity was not a recognised condition in the UK.
He added it was "not a matter for the court to say what the decision would have been if the authority went about its decision correctly".
Direct payments are given to individuals by councils so they can buy help or services instead of having them supplied by the authority.
Mr Lloyd lives in the back garden of a privately-owned home in Thornhill and has refused alternative accommodation and a portable toilet, the court heard.
He urinates in plastic bottles and defecates in bed pans lined with black bin bags.
He has no running water and an insurance company deemed the house uninhabitable after flooding and the discovery of asbestos - although that has since been removed.
When council workers went to collect Mr Lloyd's waste in May, they found 90 litres of urine and 40 bags of faeces.
He has meals delivered five times a week, including supplies for the weekend on a Friday. As he has no fridge, he has to have food which will not go off.
Representing Mr Lloyd, Christian Howells said: "Where he lives has to be adapted because the symptoms are very real to him."
He described the situation as "chicken and egg" - Mr Lloyd wants the money to pay for help to improve his day-to-day life but the council felt he was not in a position to be given the funds.
Rebecca Stickler, for the council, said: "The housing department has offered housing in a field and various properties and he has refused them all.
"It can't possibly be the case that a local authority should be compelled to use the public purse to pay money to someone who they know cannot manage that payment."
Mr Lloyd's private landlord has begun eviction proceedings in county court, he is estranged from his family and the council recognises him as homeless.
Speaking after the hearing, councillor Susan Elsmore, cabinet member for social care, health and wellbeing, insisted the council had never said Mr Lloyd was not capable of managing money.
"The council's role as a social services authority is to ensure that individuals have the opportunity to benefit from an appropriate assessment of their needs, and any services that may be necessary to meet them," she said.
"The council maintains that it has acted in keeping with its responsibilities, and in relation to this case that it has acted in good faith."
Swindon Town have fined three players after they appeared to inhale nitrous oxide in a live social media video.
[NEXT_CONCEPT]
A man who says he is allergic to electricity should not have had his payments from Cardiff council refused, a judge has ruled.