WhatsApp and Telegram have web version, while Signal needs to be downloaded
Both WhatsApp and Telegram apps can be accessed on a laptop/PC using a web browser via web.whatsapp.com and web.telegram.org. However, Signal users need to download the software based on the operating system they are using.
Telegram works natively across platforms; WhatsApp and Signal desktop apps are just an extension
Telegram is not dependent on mobile app to run on PC/laptop. Users can sign up and start using it on any platform (both laptop/PC and mobile). While for WhatsApp and Signal it is mandatory to have the mobile app. Also, the users’ smartphones should be connected to the internet all the time for users to use WhatsApp and Signal on desktop.
Signal and Telegram are available across platforms including Linux. WhatsApp does not support Linux operating system
WhatsApp is not available on Linux operating system, while the other two can run on Linux-based systems as well.
Telegram and Signal users can take calls while using these platforms on PC; WhatsApp still does not support desktop calls
Both Telegram and Signal users can make calls while using these apps on PCs/laptops. WhatsApp is beta testing desktop calls.
WhatsApp, Telegram and Signal support contact sync across platform
All the three instant messaging apps support contact and message sync on their desktop version.