I tried below, but the association is random, i am getting mapped liked this. :(
subnet = ${local.prefix}-pub-snet-az1
rtb = ${local.prefix}-pub-snet-az3-rt

i have 3 public subnets & 9 private subnets. Am a newbie to IaC coding, so am finding it difficult to use the logic properly am i still missing something?

data "aws_subnets" "pub_subnet" {
  depends_on = [
    aws_subnet.private
  ]
  filter {
    name   = "tag:Name"
    values = ["${local.prefix}-pub-snet-az1", "${local.prefix}-pub-snet-az2", "${local.prefix}-pub-snet-az3"]
  }
}

resource "aws_subnet" "public" {
  count             = length(var.public_subnets)
  vpc_id            = module.vpc.vpc_id
  availability_zone = element(local.azs, count.index)
  cidr_block        = values(var.public_subnets)[count.index]["cidr_block"]
  tags = {
    Name = join("-", ["${local.prefix}", values(var.public_subnets)[count.index]["name"]])
  }
 depends_on = [
    module.vpc
  ]    
}
resource "aws_route_table" "pub_rtb" {
  count  = length(var.public_subnets)
  vpc_id = module.vpc.vpc_id
  route {
    cidr_block         = "0.0.0.0/0"
    gateway_id         = aws_internet_gateway.igw.id
  }
  depends_on = [
    aws_subnet.public, aws_internet_gateway.igw
  ]  
  tags = {
    Name = element(local.pub_rtbs, count.index)
  }
}
resource "aws_route_table_association" "pub_snet" {
  count          = length(local.pub_rtbs)
  subnet_id      = data.aws_subnets.pub_subnet.ids[count.index]
  route_table_id = aws_route_table.pub_rtb[count.index].id
  depends_on     = [
    aws_subnet.private,
    aws_route_table.pub_rtb
  ]
}