I was trying to patch service account to rolebinding but when i ran command for patch, it replaced whole subject field in rolebinding yml. here i show my existing config and command that i executed for expected output

command to patch :

kubectl patch rolebinding test-team-binding  --patch "$(cat patch-file.yml)" 

patch-file.yml:

subjects:
- kind: ServiceAccount
  name: user3
  namespace: test-namespace

rolebinding.yml:

apiVersion: rbac.authorization.k8s.io/v1
kind: RoleBinding
metadata:
  creationTimestamp: "2020-09-08T11:24:54Z"
  managedFields:
  - apiVersion: rbac.authorization.k8s.io/v1
    fieldsType: FieldsV1
    fieldsV1:
      f:metadata:
        f:annotations:
          .: {}
          f:kubectl.kubernetes.io/last-applied-configuration: {}
      f:roleRef:
        f:apiGroup: {}
        f:kind: {}
        f:name: {}
      f:subjects: {}
    manager: kubectl
    operation: Update
    time: "2020-10-06T07:37:58Z"
  name: test-team-binding
  namespace: test-namespace
  resourceVersion: "45697451"
  selfLink: /apis/rbac.authorization.k8s.io/v1/namespaces/test-namespace/rolebindings/test-team-binding
  uid: b602b333-4ee8-4601-8c75-f3707bb19d68
roleRef:
  apiGroup: rbac.authorization.k8s.io
  kind: Role
  name: test-team
subjects:
- kind: ServiceAccount
  name: user1
  namespace: test-namespace
- kind: ServiceAccount
  name: user2
  namespace: test-namespace

expected output:

subjects:
- kind: ServiceAccount
  name: user1
  namespace: test-namespace
- kind: ServiceAccount
  name: user2
  namespace: test-namespace
- kind: ServiceAccount
  name: user3
  namespace: test-namespace

result output:

subjects:
- kind: ServiceAccount
  name: user3
  namespace: test-namespace