Não consigo instalar minhas impressoras HP Deskjet 3820 e 3740 no meu computar que roda o Windows 7. E não considero justo ter que jogá-las no lixo. Preciso de drives para instalá-la e utilizá-las normalmente.

In the future - you might choose to post on the Portuguese forums.
What you said, roughly translated into English:

I can not installmyHPDeskjet 3820and3740on mycomputer thatruns Windows 7.Considering throwing themin the trash.Driversneeded to install and use themnormally.

Hp 3820 Printer Software

All hardware support (including device drivers and software to properly utilize said hardware in *any* operating system) comes from the hardware manufacturer of said product.
For the HP Deskjet 3820 Color Inkjet Printer, start here:
Software & Driver Downloads - HP Deskjet 3820 Color Inkjet Printer
Choose the proper operating system and architecture to see what drivers HP offers for the HP Deskjet 3820 Color Inkjet Printer. For Windows 7, HP has chosen to actually *not* support this printer, but links you to this:
Installing the Printer with an Alternate Driver in Windows 7 for a USB Cable Connection

... and tell you to use the Windows Update option when trying to install the printer via the Add Printer wizard and allow the list of available printers to be downloaded and choose the 'Deskjet 970cse' as the driver to utilize for the HP Deskjet 3820 Color Inkjet Printer.
For the other device, the HP Deskjet 3740 Color Inkjet Printer, you might start here:
Software & Driver Downloads - HP Deskjet 3740 Color Inkjet Printer
Choose the proper operating system and architecture to see what drivers HP offers for the HP Deskjet 3740 Color Inkjet Printer. For Windows 7, HP has chosen to actually *not* support this printer and links you to this:
The Driver for Your Product Is Included in the Windows 7 Operating System

Network Adapter Driver Windows 7 Download

View and Download HP 3845 - Deskjet Color Inkjet Printer user manual online. HP Deskjet 3840 Printer series - (Macintosh OS X) User's Guide. 3845 - Deskjet Color Inkjet Printer Printer pdf manual download.

... which tells you that the driver you need is some generic one they gave to Microsoft when Windows 7 was released. If I were you, I would not attempt to install this printer until after you get the other one installed. Chances are it will not work (after all, HP pretty much abandoned it) - but it is a better chance after you have downloaded the updated set of drivers because of the other printer.
In the end - you may well be better off throwing out the printers and purchasing a new printer (or two - since you seem to have two now) and getting one that is Windows 8 compatible - all but insuring you three or so years of compatibility.

Hp Printer Drivers For Windows 10

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Hp 3820 Driver Windows 7 Download