Also keep in mind, that the v3 API allows multiple InfoWindows on the map. If you intend to have just one InfoWindow visible at the time, you should instead use a single InfoWindow object, and then open it and change its content whenever the marker is clicked (Source).
Google Maps API Multiple Markers with InfoWindows. In my previous post I explained How can I add a place in Google Map. Here in this post I will show you how to add multiple markers with info window in asp.net web application. Find the source code below:-. Placing multiple markers on a Google Map.