And it would become Finland all over again.
Except this time the Germans are taking part too.
If Stalin invades Romania, the those troops will not have any defensive advantages, they will be in hostile territory and if the Germans can encircle them properly then they are toast.
Can't fix an army in 6 months, especially one of that size.
This scenario in 1941 is that the VVS since the 30th of may 1941 has been planning a bombing run on the oil fields and use all bombers that can hit the targets.
Many scenarios going around
This thread is what the USSR can do if Stalin believes the intelligence with a start date of 30th of may 1941
These were my suggestions for this scenario.
The air force is set on full alert, and starts shooting down German air reconnaissance then the Germans will have less information of Soviet deployments. In addition to that, unlike the OTL the Soviet air force will not be caught unprepared and have 2,000 airplanes destroyed on the first day mostly on the ground. The pilots will also have more practical training by flying these 3 weeks, and the mechanics and the whole systems surrounding the air force will have more practical experience.
The soldiers on the ground, can be ordered to dig in and dig deep, thereby decreasing the amount of damage the initial attack does because the soldiers now have better prepared positions.
Some soldiers can be moved back to create a deeper defense.
3 weeks may allow the USSR to move back its soldiers, except some boarder guards, perhaps 20 km or more from the border, the further back they are and the deeper they have dug in, the less the initial bombardment will damage them.
Soldiers that are not fully equipped can be sent back the Stalin line and start reactivating the defenses and/or to the east of the Dnieper river and start digging in there.
As many tankers as possibly and especially those who have not fully be trained, can be given full day training courses, far from the border, and tanks that do not have their guns bored can be used for training further back.
The army can be given orders to immediately shot back if any foreign soldiers cross the border, unlike the OTL where there initially was confusion.
The Soviet generals can be given more leeway how to deploy, unlike the OTL where Stalin had demanded the deployment in a certain way.
Some industries near the border can be evacuated and moved back.
And obviously the Red Air force can be give orders to bomb the Romanian oil fields, which will tremendously hurt the axis military and the German economy, and may cause the whole invasion to falter.
The bombing could be ordered to start when the invasion starts, or maybe even do a preemptive bombing, but a preemptive attack may be used as a valid reason to launch the attack, even though Germany was planning it all along.
The more the oil fields are damaged, the more Germany will be hurt, and may even collapse.