SELECT MA.MachineID, MN.MachineName, L.LeaseNumber
FROM MachineAudit MA 
     INNER JOIN MachineName MN ON MA.MachineID = MN.MachineID
     INNER JOIN LeaseInfoImport L ON L.[computerName] = MN.[machineName]

UPDATE MachineAudit
SET LeaseNumber = L.LeaseNumber
FROM MachineAudit MA 
     INNER JOIN MachineName MN ON MA.MachineID = MN.MachineID
     INNER JOIN LeaseInfoImport L ON L.[computerName] = MN.[machineName]

create table MachineName (machineName varchar(50), machineID int)
insert into MachineName values ('PC1', 1)
insert into MachineName values ('PC2', 2)
insert into MachineName values ('PC3', 3)

create table MachineAudit (machineID int, leaseNumber int)
insert into MachineAudit values (1, null)
insert into MachineAudit values (2, null)
insert into MachineAudit values (3, null)

create table LeaseInfoImport (computerName varchar(50), leaseNumber int)
insert into LeaseInfoImport values ('PC1', 11)
insert into LeaseInfoImport values ('PC2', 12)
insert into LeaseInfoImport values ('PC3', 13)



update MachineAudit set leaseNumber = b.leaseNumber
from MachineAudit a,
(select a.machineID, b.leaseNumber from MachineName a, LeaseInfoImport b
where a.machineName = b.computerName) b
where a.machineID = b.machineID

SELECT MA.MachineID, MN.MachineName, L.LeaseNumber
FROM MachineAudit MA 
     INNER JOIN MachineName MN ON MA.MachineID = MN.MachineID
     INNER JOIN LeaseInfoImport L ON L.[computerName] = MN.[machineName]