Robert Paterson, 44, Sean McGregor, 42, and Donald Stone, 39, allegedly plotted to kill HMP Edinburgh officer Ian Duff between November 2022 and February 2023.
Court papers state the trio acted with others by engaging in conversations, texts and messages on social media.
They are said to have met at the prison and agreed to pass on details of the conspiracy to each other.
The charge states the trio discussed the acquisition of a car, firearm and ammunition.
They are claimed to have then acquired a motor vehicle.
The need for a driver was allegedly discussed and there were conversations on who it would be.
A firearm or firearms were said to have been acquired as well as pictures of them sent between those involved.
The images are claimed to have been examined to try and identify the make and model to get the suitable calibre of ammunition to be used.
Others were allegedly contacted to acquire ammunition of various calibres in order to check their compatibility.
It is claimed there were arrangements for the collection and transfer of ammunition in their possession and control to check the compatibility.
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The three men face a separate charge of being involved in serious organised crime at locations in Edinburgh, Dumbarton, Airdrie, Bathgate, Cumbernauld, Coatbridge and Ayrshire.
The charge spans between January and February 2023.
It is claimed they were involved in the collection, packaging, storing, transport and supply of controlled drugs.
Arrangements were allegedly made for Stone to receive a certificate for a motorbike course without training or tests.
This is claimed to have been done in order to facilitate the supply of controlled drugs.
Vehicles were said to have been arranged to transport and supply drugs to others.
They are also claimed to have carried out assault and robbery of drugs as well as equipment from others.
The trio are stated to have discussed and exchanged messages to coordinate the robbery of drugs, watches, sums of money and arrange a vehicle as well as a driver for it.
It is claimed they tried to arrange the supply of drugs to HMP Edinburgh.
This was allegedly done by identifying others to deliver the drugs and serving prison officers who would receive them.
Photographs of those identified are said to have been exchanged.
Methods of packaging, concealing and passing the drugs to avoid detection from staff were also allegedly discussed.
There was said to have been a conversation relating to acquiring a knife or handgun.
Paterson faces a single charge of directing McGregor and Stone to supply drugs which includes into HMP Edinburgh between January and February 2023.
Paterson further claimed to have a number of phones and SIM cards while in HMP Edinburgh.
It is claimed he used the phones to repeatedly make and receive calls as well as text messages.
The three men pleaded not guilty today at the High Court in Glasgow.
A further hearing has been set for June this year.
