The foreign exodus from Uganda is now in its third day at this border point, but for a while today the chief excitement was what might be called the great beer accident.

Kenyans here have become quite blase to the drama of hundreds of foreigners fleeing Uganda.

What did cause excitement was the collision involving two boys on the bridge over the muddy Malaba Rive, a small stream separting Kenya from Uganda.

A teen-ager in shorts and T-shirt riding downhill hell-for-leather, headed toward Uganda on his bicycle, whose luggage rack was loaded with three cases of Kenyan beer, smashed into a boy pushing an empty wheelbarrow.

The bicyclist was supplying a Ugandan merchant with beer. The other boy had rented out his wheelbarrow for the transport of travelers' belongings in the half-mile-long no-man's-land separating the Kenyan and Ugandan customs and immigration huts.

In the collision, the cyclist was knocked out cold, his face was badly cut, and his precious bottles of beer for the most part lay smashed in the sun.

The Ugandan police rushed down to inspect the resulting melee, thereby causing consternation in the crowd of Kenyans who took to their heels rather than risk the wrath of President Idi Amin's minions.

As for the hundred or so foreigners, theirs was the now familiar story of Kampala's ordeal alleviated by a relatively quiet night before they set out from the capital, Kampala, defended solely by Libyan troops, to negotiate nine roadblocks on their way to the border.

The happiest lot were about 70 Soviet diplomats evacuated in three Mercedes buses rented in Nairbbi. Three thoughtful Nairobi-based Soviet diplomats in business suits had also arranged a backup truck for their luggage and managed to persuade the Kenyan authorities to allow the evacuees through without making them disembark from the buses or go through customs and immigration for malities.

One Russian, told that American diplomats were present, carefully photographed them.

The Soviets also got preferential treatment from the Ugandans, much to the disgust of an Italian who said they got "good treatment because they are allies."

He and a half dozen other Italians were not so lucky.

They complained that the Ugandan border officials had confiscated their car and forced them to lug their Gueeibags, tennis rackets and other luggage in the hot sun across the no-msn's land. The wheelbarrow boy apparently had to pass up customers because of his accident.

A group of Asians, remnants of the once 40,000-strong community which Amin expelled in 1972, had their luggage gone through with a fine Kenyan customs comb, but they did not complain.

They, too, had their cars confiscated. Such too, was the apparent fate of Nigerian diplomats who were seen remonstrating vociferously with the admant Ugandan officials.

Three young British diplomats from Nairobi, dressed in khaki shorts in best colonial tradition, manned a stand with cold beer and soft drinks for British refugees, but none arrived.

The foreigners were all but lost in the milling African market-day throng of that was busy trading maize, meal and bananas from Uganda for Kenyan goods in short supply in Uganda.

Money-changers on the bridge did a brisk business. The going rate for the theoretically equally valuable Kenyan and Ugandan shillings was 11 to 1 in favor of the Kenyan currency, up from 10 to 1 yesterday.

If anything, the Amin regime's flickering fortunes were marginally better today, but perhaps the news had not yet reached Malaba.