LINQ múltiples uniones con múltiples condiciones

Intenta poner la constante en una condición separada...

from t1 in dbo.Table1
where t1.[Type] == 3 // <--- PUT THIS ONE HIGHER
join t1Parent in dbo.Table1 on t1.ParentId equals t1Parent.Id
join t2 in dbo.MappingT1T3 on t1Parent.Id equals Id = t2.ExternalId
where (int)t2.[Type] == 1 // <--- SEPARATE CONDITION
join t3 in dbo.Table3 on t2.ForeignId equals t3.Id;