Is there a premade triangle asset?

There is no builtin triangle primitive. Luckily the Unity Editor is easy to extend. Put this script somewhere in your project folder and a new "Triangle" button should pop up in the GameObject/3D Object menu.

 #if UNITY_EDITOR
 using UnityEngine;
 using UnityEditor;
 #endif
 
 public class TrianglePrimitive {
     #if UNITY_EDITOR
     private static Mesh CreateMesh() {
         Vector3[] vertices = {
             new Vector3(-0.5f, -0.5f, 0),
             new Vector3(0.5f, -0.5f, 0),
             new Vector3(0f, 0.5f, 0)
         };
 
         Vector2[] uv = {
             new Vector2(0, 0),
             new Vector2(1, 0),
             new Vector2(0.5f, 1)
         };
 
         int[] triangles = { 0, 1, 2 };
 
         var mesh = new Mesh();
         mesh.vertices = vertices;
         mesh.uv = uv;
         mesh.triangles = triangles;
         mesh.RecalculateBounds();
         mesh.RecalculateNormals();
         mesh.RecalculateTangents();
         return mesh;
     }
 
     private static GameObject CreateObject() {
         var obj = new GameObject("Triangle");
         var mesh = CreateMesh();
         var filter = obj.AddComponent<MeshFilter>();
         var renderer = obj.AddComponent<MeshRenderer>();
         var collider = obj.AddComponent<MeshCollider>();
 
         filter.sharedMesh = mesh;
         collider.sharedMesh = mesh;
         renderer.sharedMaterial = AssetDatabase.GetBuiltinExtraResource<Material>("Default-Material.mat");
 
         return obj;
     }
 
     [MenuItem("GameObject/3D Object/Triangle", false, 0)]
     public static void Create() {
         CreateObject();
     }
     #endif
 }