How do I plan crews and equipment across several sites at once?

Short answer

Put every active site on a single calendar, with each one's stages and deadlines.
Assign crews to stages, not to free days — a stage has a duration and predecessors.
Flag shared equipment (crane, excavator, mixer truck) as single resources, so it can't be promised in two places.
Check for clashes before you confirm a new deadline to a client.
Reschedule from the same screen when a stage slips, so you see the knock-on effect on the other sites at once.

Labour Code, Art. 119(1) — the employer must keep, at the workplace, a daily record of hours worked by each employee with start and end times, and present it to labour inspectors on request. Across several sites, the record is kept per work location.
Government Decision 300/2006 on minimum health and safety requirements for temporary or mobile construction sites — each site has its own health and safety plan, drawn up by the contractor within 30 days of being awarded the work.
Law 319/2006 on occupational health and safety, with the implementing rules in Government Decision 1425/2006 — workplace induction is required for each site a worker is moved to.
Lifting equipment such as cranes and platforms is subject to ISCIR authorisation and periodic inspection — equipment with a lapsed inspection cannot be scheduled on any site.

Example: a company with 3 active sites and 4 crews owns one crane. The structural stage on site A needs it for 12 days, site B for 9 days, over the same period. Planned as a single resource, the two stages are staggered and site B's deadline is renegotiated before signing, not after.
Example: the 5-person finishing crew is assigned 6 days to a 78 m² apartment, then moved to the second site. The hours record stays separate per work location, and induction is repeated on the move.

Planning people and forgetting equipment — two sites end up waiting for the same crane.
Keeping the plan in spreadsheets, one file per site: clashes only become visible on the ground.
Promising a client a deadline before checking which crew is actually free then.
Moving people between sites without induction at the new location and without separating the hours record.

Every active site, with its stages and deadlines, on a single project calendar.
Crews and equipment are assigned to stages, and the same resource booked on two sites is flagged.
Delay alerts per stage, so you see the effect on the other sites before it propagates.
The daily journal shows, per site, which crew was present and which equipment actually worked.