* Three smaller unions agree to return to work
* Pressure on biggest transport union to follow
* Threat of truck driver strike spreading to rail, ports
* Fuel, consumer good, cash deliveries disrupted
(Recasts with smaller unions agreeing deal)
By Agnieszka Flak and Wendell Roelf
JOHANNESBURG/CAPE TOWN, Oct 9 (Reuters) - About 15,000 striking South African truckers agreed to return to work on Wednesday, easing pressure on Africa's biggest economy where two weeks of labour unrest in the transport sector have hit supplies of fuel, cash and consumer goods.
The decision on Tuesday by three small unions puts pressure on the biggest labour group, the South African Transport and Allied Workers Union (SATAWU), which represents about 28,000 workers, to reach a deal and suspend its calls to widen the strike to ports and rails.
It also eased investors' concerns about widening strikes that could slow growth.
The rand , which fell to 3-1/2 year lows against the dollar on Monday on worsening investor sentiment about labour strife, firmed immediately after the news truckers would return to work, hitting a session high of 8.735.
"Three of the unions have agreed to suspend strike action," a spokeswoman for the employers association said. It was still in talks with all groups to hammer out a final deal.
Large parts of the mining sector, responsible for about 6 percent of gross domestic product, have been brought to a standstill in the last two months by wildcat strikes by more than 75,000 miners - about 15 percent of its workforce.
Almost 50 people have been killed in the current labour strife - 34 of them by shot dead by police on Aug. 16 in the deadliest security incident since the end of apartheid in 1994.
President Jacob Zuma's ruling African National Congress has been criticised for letting the strikes spread. Moody's ratings agency downgraded South African government bonds a notch last month, saying ineffectual governance was posing long term economic risk.
Thousands of striking truck drivers have taken to the streets in often violent protests, demanding annual wage increases of 12 percent for two years - more than double the inflation rate.
Employers have offered a total 18 percent pay rise over that period. It held wage talks with SATAWU on Tuesday.
"We are willing to compromise on our demands, but only as long as the employers do the same," said Vincent Masoga, spokesman for SATAWU. He was not immediately able to comment on the deal struck by the smaller unions.
An employers body said last week the freight industry was losing around 1.2 billion rand ($135 million) in turnover each week due to the strike.
Affected companies include logistics groups Imperial Holding , Super Group , Grindrod , Barloworld
and Bidvest . If the protests expand to rail and ports, they would hit exports of coal and other minerals.
The mining strikes look set to knock already shaky economic growth in the world's top platinum-producing state. South Africa is also a major supplier of gold, coal and iron ore.
Local media reports suggested the truckers' strike was also having adverse effects on Zimbabwe, which receives a steady supply of goods over road from its larger neighbour.
On Monday, a local government workers' union said it also planned a protest in the next few days in a first sign of the labour unrest spreading into the public sector. ($1 = 8.8828 South African rand)
(Additional reporting by Xola Potelwa; Editing by Jon Herskovitz and Andrew Heavens)
Keywords: SAFRICA STRIKES/