Adding multiple SCNNode(s) at the same time

Your code works fine (a position was a problem):

import SwiftUI
import ARKit

struct ContentView : View {
    
    @State var addBox: Int = 3
    @State var reset: Bool = false
    @State var node: SCNNode = SCNNode()
    
    var body: some View {
        return SceneKitView(addBox: $addBox,
                             reset: $reset,
                              node: $node).edgesIgnoringSafeArea(.all)
    }
}

struct SceneKitView: UIViewRepresentable {
    
    let arView = ARSCNView(frame: .zero)
    let config = ARWorldTrackingConfiguration()
    @Binding var addBox: Int
    @Binding var reset: Bool
    @Binding var node: SCNNode
    
    fileprivate func addShapes() {
        let letBall1Geo = SCNSphere(radius: 0.05)
        letBall1Geo.firstMaterial?.diffuse.contents = UIColor.red
        letBall1Geo.firstMaterial?.specular.contents = UIColor.white
        let ball1 = SCNNode(geometry: letBall1Geo)
        ball1.position = SCNVector3(0, 0, -1)
        self.arView.scene.rootNode.addChildNode(ball1)
                
        let letBall2Geo = SCNSphere(radius: 0.05)
        letBall2Geo.firstMaterial?.diffuse.contents = UIColor.green
        letBall2Geo.firstMaterial?.specular.contents = UIColor.white
        let ball2 = SCNNode(geometry: letBall2Geo)
        ball1.position = SCNVector3(0, 0.2, -1)
        self.arView.scene.rootNode.addChildNode(ball2)
        
        let stickGeo = SCNCapsule(capRadius: 0.05, height: 0.1)
        stickGeo.firstMaterial?.diffuse.contents = UIColor.blue
        stickGeo.firstMaterial?.specular.contents = UIColor.white
        let stick = SCNNode(geometry: stickGeo)
        stick.position = SCNVector3(0, 0.4, -1)
        self.arView.scene.rootNode.addChildNode(stick)            
    }

    func makeUIView(context: Context) -> ARSCNView {
        arView.scene = SCNScene()
        arView.autoenablesDefaultLighting = true
        arView.debugOptions = [.showFeaturePoints, .showWorldOrigin]
        arView.session.run(self.config)
        return arView
    }
    
    func updateUIView(_ uiView: ARSCNView, context: Context) {
        if addBox > 0 {
            self.addShapes()
        }
    }
}