alxsanchez,
You can do one of two things. Actually let me tell you what can't be done. Then I will show how to do it correctly.
Your first example has 3-7 for the days. This close to correct except there is no "7". It would be "3-0". 3 is Tuesday and 0 is Sunday. The days start at 0 for Sunday through 6 for Saturday.
Your second example cannot be done. Mixing "," and "-" is not allowed on one of the 5 parameters. You can use "5,15" for minutes and then use "8-6" for Hours, but not in the same parameter.
Look at this example.
# minutes (0-59) hours (0-23) dates (1-31) months (1-12) days (0-6)
0,15,30,45 8-5 * * 1-5 /usr/bin/somescript
For your example you can either do:
0 5 * * 0, 2, 3, 4, 5, 6 /usr/bin/somescript
or
0 5 * * 2-0 /usr/bin/somescript
It might help you some more to read the man page for cron.
Just type "man cron" at the prompt.
Also do a crontab -l to see your actual real crontab on your system.
Hope this helps.