That emerged last night after a Shadow Cabinet meeting was given a strong warning by Mr Blair that the party in the country would not forgive a continuation of the recent spate of high profile conflicts between its members in the run-up to the general election.
The three men - in effect the party's most three most prominent modernisers - were said to have agreed that Mr Brown and Mr Mandelson should meet more, rather than less, frequently to co-ordinate their strategic role over the next year.
At last night's Shadow Cabinet meeting Mr Blair warned his colleagues that "gossip" about internal differences within the party would be used to highly-successful effect by the party's political opponents.
Earlier, Mr Blair had been forced to dismiss as "Westminster tittle- tattle" renewed reports of splits at the top of the Labour Party after his deputy, John Prescott, appeared to challenge Mr Brown's authority.
Mr Prescott's speech on Monday followed the weekend exposure of the two-year rift between Mr Brown and Mr Mandelson, one of Mr Blair's closest confidants.
With one Labour source saying that Mr Brown and Mr Mandelson had "learnt a lesson" from the past few days, the two men are said to have resolved not to let their opponents - inside as well as outside the party - "drive a wedge between them". Mr Brown insisted on the Today programme on Radio 4 yesterday that he and Mr Mandelson "talk socially and we talk politically. Peter Mandelson and I work so closely on the election planning that we meet every day at 9 o'clock. Peter is one of the most brilliant election strategists who has done a tremendous amount for the party."
The important issue arising from the turmoil of the past few days is the state of Mr Blair's relationship with Mr Brown. Theirs was one of the most successful partnerships in recent British politics, with Mr Blair in the junior role for the first 10 to 11 years. The relationship survived the trauma of the private struggle between them for the Labour leadership in the days after John Smith's death, two years ago this week.
But it changed irretrievably then, and has adjusted as Mr Blair has energetically asserted his authority over the party.
Some of Mr Brown's opponents in the Labour Party suggest that Mr Blair has been "bounced" by his shadow Chancellor into backing some of his recent controversial initiatives. But this is certainly untrue in both recent cases which have aroused resentment among Labour MPs and party members.
One was Mr Brown's threat to cut benefits for young people who refuse to take up the opportunities a Labour government would offer, sprung on a Westminster news conference in November. The second was the "review" of child benefit for 16- to 18-year-old students, leaked to the Times and the Daily Mirror in advance of Mr Brown giving the John Smith Memorial Lecture last month.
In both cases, Mr Brown had the full support in advance of the Labour leader.
Mr Blair and Mr Brown united yesterday to deny that there were any splits. Speaking at the launch of policies on youth unemployment, Mr Blair said: "I don't think I have read so much drivel as has been talked about in the last few days. What we have actually witnessed over the last few days is a whole load of fevered nonsense."
Earlier, Mr Brown denied that Mr Prescott's warning against a "super Treasury" under a Labour government was aimed at curbing his ambitions.
The tensions in the party are likely to continue in the run-up to the election, but all the evidence suggests that Mr Brown's line has Mr Blair's full support.