The swearing-in will automatically bring an end to Madrid's direct rule over the wealthy, northeastern region imposed in October after a failed bid to break from Spain.
The trio, all former ministers in the regional government of ousted Catalan leader Carles Puigdemont, faced a European arrest warrant issued against them in March by Madrid.
Judge Carmen Lamela of the National Court, which deals with high-profile political and financial cases, said in her ruling there was evidence that former Mossos d'Esquadra chief Josep Lluis Trapero was part of a "criminal organisation" that sought to break Catalonia away from Spain.
Local court rules Puigdemont must remain in custody pending procedures on whether he will be extradited to Spain
Ex-Catalan leader supports president of Catalan National Assembly as alternative presidential candidate
The three include deposed Catalan vice-president Oriol Junqueras, as well as the leaders of two grassroots separatist groups: Jordi Sanchez of the ANC and Jordi Cuixart of Omnium Cultural.
His detractors have called the results of the December 21 election, which saw separatist parties hold on to their majority in the Catalan parliament, a "fiasco" and a "rout" for Rajoy, in power since 2011.
Belgian judge defers decision to Dec. 4, process to extradite former separatist politicians to Spain could take three months
Separatists have called on supporters to match the annual turn-out of Catalonia's national day, when hundreds of thousands gather in the city. The demonstration is due to take place from 17:00 (16:00 GMT).
Carme Forcadell will be released on $175,000 bail
A judge ordered the five, who fled Spain earlier this week after being accused of rebellion and sedition, not to leave Belgium until their extradition case is heard.
Carles Puigdemont in Belgium after Spanish attorney general calls for ex-Catalan president to face charges
Barcelona’s streets filled with protesters, waving Spanish flags, concentrating against Friday’s independence declaration
As thousands rallied in Madrid in support of Spanish unity, Carles Puigdemont's office said he would make a speech at the seat of the regional government at 2:30 pm (1230 GMT).
EU's heavyweight Germany says sovereignty and territorial integrity of Spain remain inviolable
Pro-independence associations call for protests Tuesday against decision to jail the two leaders
Whether or not Catalan president Carles Puigdemont will follow through on his threat to announce a full breakaway -- defying the central government and Spanish courts -- is still a mystery.
Spanish PM says that “there was not a self-determination referendum in Catalonia today”
Police have orders to remove students and parents at 6 a.m. Sunday to block the vote
Clinging to independence bid, Catalan government says its police will 'never give up exercising powers that are its own'
Thousands took to the streets of the Mediterranean seaside city on Wednesday after police detained key members of the team allegedly organising the vote slated for October 1 in a region deeply divided over independence.