Inline Property Attribute

The Inline Property is used to place the contents of a type next to the label, instead of being rendered in a foldout. 

public Vector3 Vector3;

public Vector3Int MyVector3Int;

[InlineProperty(LabelWidth = 13)]
public Vector2Int MyVector2Int;

[Serializable]
[InlineProperty(LabelWidth = 13)]
public struct Vector3Int
{
    [HorizontalGroup]
    public int X;

    [HorizontalGroup]
    public int Y;

    [HorizontalGroup]
    public int Z;
}

[Serializable]
public struct Vector2Int
{
    [HorizontalGroup]
    public int X;

    [HorizontalGroup]
    public int Y;
}