Even though the files from your 750D are in the .cr2 format, the data in a raw file is always sensor specific. That is, each different sensor design must be demosaiced/converted differently to get the same standardized results.
LR3 is a while back. The latest version of Adobe Camera Raw (which is used by LR to demosaic raw files) that is compatible with LR3 was released a long time before the sensor in the Canon 750D was developed. It is no surprise that LR3 can't handle the raw files from your new camera. Since Adobe has chosen to not support newer cameras with older versions of ACR/LR, if you wish to work with the .cr2 files created by your 750D you have a few options:
- Update your version of Lightroom and Adobe Camera Raw to versions that can support the Canon EOS 750D.
- Use Canon's own Digital Photo Professional supplied with every Canon DSLR. Updates and even new versions are always free for owners of Canon EOS cameras.
- Convert the /cr2 files from your 750D to the DNG format and edit them with LR3. Be aware that some information in the .cr2 file is discarded when converted to .dng. That information may or may not be important to you.
So just what is going on under the hood in Lightroom, Photoshop, and Adobe Camera Raw?
As Adobe updates ACR to support newer cameras, compatibility with the newer versions of ACR also requires newer versions of Photoshop and Lightroom. What version of Adobe Camera RAW you need depends on exactly which camera made the raw files in question and which version of Photoshop or Lightroom you are using. Once you know what the earliest version of ACR is that supports your particular camera, then you can find the earliest versions of Photoshop or Lightroom that support that version of ACR. For newer versions of Photoshop or Lightroom you may also need a newer version of ACR than the oldest one that supports your camera.
Can I download a later version of Lightroom from somewhere?
Here is a link with a comprehensive set of instructions from Adobe that addresses the issue you are encountering.
Here is a link that lists camera models currently supported by Adobe products via Adobe Camera Raw and the earliest version of ACR that supports raw files from each camera. The chart also lists the earliest version of Adobe Lightroom that supports raw files from each camera (via the listed version of ACR).
From the charts linked above we see that the earliest version of ACR that supports the Canon EOS 750D is v9.0. ACR v9.0 requires either Lightroom CC v1 or later, Lightroom Classic CC v2015.0 or later, Lightroom v6.0 or later, or any version of Photoshop CC (please see this link). Depending on the OS your computer is running, you might be able to use Photoshop CS6 with ACR9 (please see 'note 5' at this link).
If you choose to use Adobe DNG Convertor to convert your raw files to a DNG version supported by an older version of Photoshop, be aware that the conversion may remove some of the functionality and flexibility you would have if you used a newer version of ACR/LR/PS to work with the raw files directly. This will probably be most noticeable with regard to color and noise profiles for newer cameras used with older versions of ACR/LR/PS. Again, it all depends on which camera (and more generally, which manufacturer) the raw files are coming from.
Canon's in-house Digital Photo Professional 4 supports raw files from the 750D, as well as most other Canon DSLRs released in the past 10-15 years. They're still rolling out support for some of the oldest camera from way back in the beginning of the consumer digital camera era.
Some of the previous versions of DPP have been rightfully criticized for being clunky and limited. But version 4 is fairly full featured regarding raw conversion of images from Canon cameras. You can also edit jpegs from the same list of supported Canon cameras, but without the ability to use the raw only tools.
With global image adjustments (adjustments that affect the entire image) it does pretty much everything any other raw converter/editor does as well as a few things the others can't. Applying Canon's proprietary Digital Lens Optimizer, for example, that provides a level of lens correction for Canon lenses that goes well beyond the typical lens correction available from other applications.
What you don't get from DPP 4 is as much in the way of cataloging or the ability to add keywords to the metadata. But there are other applications that can fill in that gap.
It's free for the downloading at any of Canon's support sites in various world regions. It's my primary raw editing/conversion software.