* 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 15,000striking South African truckers agreed to return to work onWednesday, easing pressure on Africa's biggest economy where twoweeks of labour unrest in the transport sector have hit suppliesof fuel, cash and consumer goods.
The decision on Tuesday by three small unions puts pressureon the biggest labour group, the South African Transport andAllied Workers Union (SATAWU), which represents about 28,000workers, to reach a deal and suspend its calls to widen thestrike to ports and rails.
It also eased investors' concerns about widening strikesthat could slow growth.
The rand , which fell to 3-1/2 year lows against thedollar on Monday on worsening investor sentiment about labourstrife, firmed immediately after the news truckers would returnto 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 stillin talks with all groups to hammer out a final deal.
Large parts of the mining sector, responsible for about 6percent of gross domestic product, have been brought to astandstill in the last two months by wildcat strikes by morethan 75,000 miners - about 15 percent of its workforce.
Almost 50 people have been killed in the current labourstrife - 34 of them by shot dead by police on Aug. 16 in thedeadliest security incident since the end of apartheid in 1994.
President Jacob Zuma's ruling African National Congress hasbeen criticised for letting the strikes spread. Moody's ratingsagency downgraded South African government bonds a notch lastmonth, saying ineffectual governance was posing long termeconomic risk.
Thousands of striking truck drivers have taken to thestreets in often violent protests, demanding annual wageincreases of 12 percent for two years - more than double theinflation rate.
Employers have offered a total 18 percent pay rise over thatperiod. It held wage talks with SATAWU on Tuesday.
"We are willing to compromise on our demands, but only aslong as the employers do the same," said Vincent Masoga,spokesman for SATAWU. He was not immediately able to comment onthe deal struck by the smaller unions.
An employers body said last week the freight industry waslosing around 1.2 billion rand ($135 million) in turnover eachweek due to the strike.
Affected companies include logistics groups Imperial Holding, Super Group , Grindrod , Barloworld
and Bidvest . If the protests expand to railand ports, they would hit exports of coal and other minerals.
The mining strikes look set to knock already shaky economicgrowth in the world's top platinum-producing state. South Africais also a major supplier of gold, coal and iron ore.
Local media reports suggested the truckers' strike was alsohaving adverse effects on Zimbabwe, which receives a steadysupply of goods over road from its larger neighbour.
On Monday, a local government workers' union said it alsoplanned a protest in the next few days in a first sign of thelabour unrest spreading into the public sector.($1 = 8.8828 South African rand)
(Additional reporting by Xola Potelwa; Editing by JonHerskovitz and Andrew Heavens)
Keywords: SAFRICA STRIKES/