SELECT t1.Id, t1.RegistryDate,
       count(distinct t2.Id) as PlacesCount,
       sum(distinct t2.Weight) as RealWeight,
       sum(distinct t3.Weight) as GroupWeight
FROM Acts as t1
left join ActCustomerPlaces as t2 on t2.ActId = t1.Id
left join ActGroupWeights as t3 on t3.ActId = t1.Id
group by t1.Id desc;

var query = from act in context.Acts.AsNoTracking()
                            from t1 in context.ActCustomerPlaces.Where(x => act.Id == x.ActId).DefaultIfEmpty()
                            from t2 in context.ActGroupWeights.Where(x => act.Id == x.ActId).DefaultIfEmpty()
                            group new { act, t1, t2 } by act.Id into g
                            orderby g.Key descending
                            select new
                            {
                                Id = g.Key,
                                RegistryDate = g.FirstOrDefault().act.RegistryDate,
                                PlacesCount = g.Select(x => x.t1.Id).Distinct().Count(),
                                RealWeight = g.Distinct().Sum(x => x.t1.Weight),
                                GroupWeight = g.Distinct().Sum(x => x.t2.Weight)
                            };