Instead of aligning by spacers, move each part into separate view, say LeftHeader, CenterHeader, and RightHeader and use frame alignment, which gives exact separation by equal sizes, like
HStack {
LeftHeader()
.frame(maxWidth: .infinity, alignment: .leading)
CenterHeader()
.frame(maxWidth: .infinity, alignment: .center)
RightHeader()
.frame(maxWidth: .infinity, alignment: .trailing)
}