That agreement is mentioned on Wikipedia in the Leon Wilkeson entry:
"Wilkeson's death also put the group in a strange position, since an
agreement with Ronnie Van Zant's widow stated that at least three of
Skynyrd's longtime members would have to be in attendance for it to be
able to tour and issue albums under the name 'Lynyrd Skynyrd.' The
group continued on anyway, with replacement bassist Ean Evans taking
Wilkeson's longtime spot."
There seems to be another agreement described on Wikipedia under Lynyrd Skynyrd:
"The fact that the band chose to continue after the 1987 tribute tour
caused legal problems for the survivors, as Judy Van Zant Jenness and
Teresa Gaines Rapp (widows of Ronnie and Steve, respectively) sued
the others for violating an agreement made shortly after the plane
crash, stating that they would not 'exploit' the Skynyrd name for profit."
Regarding remaining members, it says:
"Since Collins, Wilkeson and Powell are now dead, Ed King unable to
tour due to ongoing heart problems, and Pyle on the outs with the
others leaves Gary Rossington as the Skynyrd standard-bearer."
"Wilkeson's death also put the group in a strange position, since an
agreement with Ronnie Van Zant's widow stated that at least three of
Skynyrd's longtime members would have to be in attendance for it to be
able to tour and issue albums under the name 'Lynyrd Skynyrd.' The
group continued on anyway, with replacement bassist Ean Evans taking
Wilkeson's longtime spot."
There seems to be another agreement described on Wikipedia under Lynyrd Skynyrd:
"The fact that the band chose to continue after the 1987 tribute tour
caused legal problems for the survivors, as Judy Van Zant Jenness and
Teresa Gaines Rapp (widows of Ronnie and Steve, respectively) sued
the others for violating an agreement made shortly after the plane
crash, stating that they would not 'exploit' the Skynyrd name for profit."
Regarding remaining members, it says:
"Since Collins, Wilkeson and Powell are now dead, Ed King unable to
tour due to ongoing heart problems, and Pyle on the outs with the
others leaves Gary Rossington as the Skynyrd standard-bearer."