The deb files for the 3.0 release were accidentally build from the RC1 sources.
As of 23 Dec they have been replaced. If you downloaded before, then please download again.
You can verify the version in the "About" page (Help menu). Or check the checksums.
The new checksums
d0f7f42781a5ef7a7c1cc7c5da35212e lazarus-project_3.0.0-0_amd64.deb
2ac1a86b059067a7b3d3ebfb2c78d879 lazarus-project_3.0.0-0_i386.deb
46fa925af5d5ff3de9c71d9552210516ba4da920 lazarus-project_3.0.0-0_amd64.deb
78c1ead9216c47fe329d489702f9a7178e56ef03 lazarus-project_3.0.0-0_i386.deb
9f0e0b830e4b2c8d91013ee076b7edf3a6040042ae66c8bf8148557ef2a5e53d lazarus-project_3.0.0-0_amd64.deb
f4d2735083247b9617d2af9ee395abeed45c8b665ff317fc57fa84fe87edc969 lazarus-project_3.0.0-0_i386.deb
The old checksums (RC1 builds):
6f9e7795dfa903fe817e1c0807b2f8b7 lazarus-project_3.0.0-0_amd64.deb
2ed4138507a9774749eb2e8442761633 lazarus-project_3.0.0-0_i386.deb
d73aaaf23e68e352016131acac82a0d07212ded4 lazarus-project_3.0.0-0_amd64.deb
417b37e12fcc220fa10d95e51464ab873843dfe5 lazarus-project_3.0.0-0_i386.deb
9e4f7d6e907bd1e035c12cd6f154ac26d46ca6accf34ba762883f9ad918a26f5 lazarus-project_3.0.0-0_amd64.deb
2719737ba7d8a7cc3e1a1f84ea9da11a1d1310f79ca4993ba20e720eedae94fb lazarus-project_3.0.0-0_i386.debThe webpage
https://www.lazarus-ide.org/index.php?page=checksums#3_0 will be updated asap.