CONFLICT between China and Britain over the so-called ''through train'' deepened yesterday when the third round of talks opened on Hongkong's 1994-95 electoral arrangements. Just an hour before the talks, Chinese Vice-Premier and Foreign Minister, Mr Qian Qichen, told a delegation of local district board members that the through train - under which legislators elected in 1995 will be allowed to serve beyond 1997 - should notbe discussed. He was quoted by the head of the delegation, Mr Wong Chi-keung, as saying that the power to vet legislators must rest with the Special Administrative Region Preparatory Committee, which would not be set up until 1996. Mr Wang Hanbin, vice-chairman of the National People's Congress (NPC), said the NPC Standing Committee could not make a decision on the through train on behalf of the Hongkong members of the preparatory committee. ''If the NPC Standing Committee pre-empts the decision of the preparatory committee, that means we do not respect Hongkong people,'' Mr Wang told the same delegation. However, Sir Robin McLaren, Britain's chief negotiator and ambassador to China, made it clear that the through train was a ''very important requirement'' of the British side. Speaking after a three-hour session, Sir Robin said: ''The Chinese side is well aware of the importance we have attached to it.'' A British source asked: ''What is the point of us holding talks with the Chinese on the electoral arrangements if the through train can't be guaranteed? ''It is doubtful that an agreement without any guarantee on the through train will be acceptable to Hongkong people,'' the source said. In Hongkong, many executive councillors and liberal legislators said the qualifications for legislators to serve beyond 1997 should be spelt out before the 1995 elections. Senior Executive Councillor Lady Dunn said the qualification of the legislators sitting on the 1997 legislature as stipulated in the Basic Law needed clarification. Another executive councillor, Professor Felice Lieh-mak, said it was a matter of practicality that the through train problem should be discussed. ''It is important for the voters to determine whether the candidates have a political future or not,'' she said. The Chairman of the United Democrats, Mr Martin Lee Chu-ming, said the through train problem should be resolved before the 1995 elections or the next batch of legislators would be obliged to consult Chinese officials before voting on sensitive questions. ''That would make a total nonsense of high autonomy and Hongkong people ruling Hongkong,'' Mr Lee said. But pro-China legislator Mr Tam Yiu-chung from the Democratic Alliance for the Betterment of Hongkong said it was not appropriate to resolve the through train problem in the Sino-British talks. It should be up to the preparatory committee to decide, he said. Liberal Party legislator Mr Allen Lee Peng-fei said whether legislators could serve beyond 1997 was not important. ''It has become an issue in the talks. I wonder whether it is a created issue,'' said Mr Lee, convenor of the Liberal Party's Preparatory Committee.