ARKit 3的身体追踪算法可以通过以下步骤解决：

1. 导入ARKit框架和Vision框架：

import ARKit
import Vision

2. 创建ARSession并启用人体追踪功能：

let session = ARSession()
let configuration = ARWorldTrackingConfiguration()
configuration.frameSemantics.insert(.personSegmentationWithDepth)
session.run(configuration)

3. 在ARSessionDelegate中实现人体追踪的回调方法：

extension ViewController: ARSessionDelegate {
    func session(_ session: ARSession, didUpdate frame: ARFrame) {
        guard let personSegmentationBuffer = frame.segmentationBuffer,
            let personSegmentationMask = try? personSegmentationBuffer.createMask(colorScheme: .gray),
            let depthMap = frame.sceneDepth?.depthMap
        else {
            return
        }
        
        let bodyTrackingRequest = VNDetectHumanBodyPoseRequest(completionHandler: handleBodyTracking)
        let bodyTrackingHandler = VNImageRequestHandler(cvPixelBuffer: depthMap, orientation: .right, options: [:])
        
        do {
            try bodyTrackingHandler.perform([bodyTrackingRequest])
        } catch {
            print("Error performing body tracking request: \(error)")
        }
    }
    
    func handleBodyTracking(request: VNRequest, error: Error?) {
        guard let observations = request.results as? [VNHumanBodyPoseObservation] else {
            return
        }
        
        // 处理每个人体姿势的结果
        for observation in observations {
            // 提取关键点
            guard let recognizedPoints = try? observation.recognizedPoints(.all) else {
                continue
            }
            
            // 处理关键点数据
            for (_, point) in recognizedPoints where point.confidence > 0.3 {
                let normalizedPoint = CGPoint(x: CGFloat(point.location.x), y: CGFloat(point.location.y))
                let unprojectedPoint = self.unprojectPoint(normalizedPoint)
                
                // 在AR场景中使用unprojectedPoint进行相应的操作
                // ...
            }
        }
    }
}

4. 在handleBodyTracking方法中，可以提取关键点的位置信息，并根据需要进行进一步的处理。在示例中，我们将提取关键点的位置，并使用unprojectPoint方法将其转换为AR场景中的3D坐标。

func unprojectPoint(_ point: CGPoint) -> SCNVector3 {
    let hitTestResults = sceneView.hitTest(point, types: .featurePoint)
    
    guard let result = hitTestResults.first else {
        return SCNVector3Zero
    }
    
    return SCNVector3(result.worldTransform.columns.3.x, result.worldTransform.columns.3.y, result.worldTransform.columns.3.z)
}

这样，你就可以在ARKit 3中使用身体追踪算法，提取人体关键点的位置信息，并在AR场景中进行相应的操作。