The legal requirement is for the business of the meeting to be served on the councillors with 3 clear days and in practice this should refer to the agenda and supporting documents. However, legally, it is only the agenda that is covered by this legislation. The law in question is the Local Government Act 1972, Section 12, Section 10 2(b) which states
Three clear days at least before a meeting of a parish council
(b)a summons to attend the meeting, specifying the business proposed to be transacted at the meeting and signed by the proper officer of the council, shall be left at or sent by post to the usual place of residence of every member of the council.
That said, it is good practice to include all the supporting documents at the same time. However, it may be the case that the Clerk is waiting for a response or information to be sent to them before they can forward it to the councillors so there is always the need for some flexibility. The same can be said for month end accounts which cannot physically be created any earlier and therefore may have to be distributed closer to the meeting than the summons....
Councillors have the right to request that the supporting documents are presented in a timely manner unless there is a genuine reason that the Clerk is unable to do so.