It seems strange that you want to create a report via SOQL. I assume you mean to create a report to export via the REST API (like Workbench).
1) You won't be able to achieve this via SOQL because SOQL statements must traverse a valid relationship. The documentation here gives more information. So you could use SOQL to get data from Order and Contact records because there is a relationship between these two objects, but you would not be able to traverse anywhere to get to the Lead object (since there is no relationship between the aforementioned objects and the Lead object).
When you are traversing between objects via SOQL, you will either be using downward or upward traversal. This is a good link to see how this can be achieved.
2) However, you will be able to achieve this via a Joined report type via the UI. As this is achievable via the UI, this doesn't require any SOQL or SOSL. The benefit here is that you can save this report and save it to a report folder and then distribute the report link to your colleagues who have a Salesforce license.
3) If you really did want to use SOQL to create a report, then you could create a relationship (lookup relationship between Lead and Contact may be preferable). So the Lead would be related to the Contact via an optional lookup, and this would mean it would be possible to traverse between those three objects. Or another way would be to have 'Order' as the junction object, allowing the Order object to be related to n number of Leads and n number of Contacts.
Hope this helps.